Fail Gradle build for Javadoc warnings

In order to catch Javadoc errors in the build, we now enable the
`Xwerror` flag for the `javadoc` tool. In addition, we now use
`Xdoclint:syntax` instead of `Xdoclint:none` in order to validate
syntax within our Javadoc.

This commit fixes all resulting Javadoc errors and warnings.

This commit also upgrades to Undertow 2.2.12.Final and fixes the
artifact names for exclusions for the Servlet and annotations APIs.

The incorrect exclusion of the Servlet API resulted in the Servlet API
being on the classpath twice for the javadoc task, which resulted in the
following warnings in previous builds.

javadoc: warning - Multiple sources of package comments found for package "javax.servlet"
javadoc: warning - Multiple sources of package comments found for package "javax.servlet.http"
javadoc: warning - Multiple sources of package comments found for package "javax.servlet.descriptor"
javadoc: warning - Multiple sources of package comments found for package "javax.servlet.annotation"

Closes gh-27480

spring-context/src/main/java/org/springframework/context/annotation/AnnotationBeanNameGenerator.java

@@ -156,7 +156,7 @@ public class AnnotationBeanNameGenerator implements BeanNameGenerator {
 	/**
 	 * Derive a default bean name from the given bean definition.
 	 * <p>The default implementation simply builds a decapitalized version
-	 * of the short class name: e.g. "mypackage.MyJdbcDao" -> "myJdbcDao".
+	 * of the short class name: e.g. "mypackage.MyJdbcDao" &rarr; "myJdbcDao".
 	 * <p>Note that inner classes will thus have names of the form
 	 * "outerClassName.InnerClassName", which because of the period in the
 	 * name may be an issue if you are autowiring by name.

spring-context/src/main/java/org/springframework/context/support/ReloadableResourceBundleMessageSource.java

@@ -315,7 +315,7 @@ public class ReloadableResourceBundleMessageSource extends AbstractResourceBased
 	/**
 	 * Calculate the filenames for the given bundle basename and Locale,
 	 * appending language code, country code, and variant code.
-	 * E.g.: basename "messages", Locale "de_AT_oo" -> "messages_de_AT_OO",
+	 * <p>For example, basename "messages", Locale "de_AT_oo" &rarr; "messages_de_AT_OO",
 	 * "messages_de_AT", "messages_de".
 	 * <p>Follows the rules defined by {@link java.util.Locale#toString()}.
 	 * @param basename the basename of the bundle

spring-context/src/main/java/org/springframework/remoting/rmi/JndiRmiClientInterceptor.java

@@ -56,7 +56,7 @@ import org.springframework.util.Assert;
  * For example:
  *
  * <pre class="code">&lt;property name="jndiEnvironment"&gt;
- * 	 &lt;props>
+ * 	 &lt;props&gt;
  *		 &lt;prop key="java.naming.factory.initial"&gt;com.sun.jndi.cosnaming.CNCtxFactory&lt;/prop&gt;
  *		 &lt;prop key="java.naming.provider.url"&gt;iiop://localhost:1050&lt;/prop&gt;
  *	 &lt;/props&gt;

spring-context/src/main/java/org/springframework/remoting/rmi/JndiRmiProxyFactoryBean.java

@@ -43,7 +43,7 @@ import org.springframework.util.ClassUtils;
  * For example:
  *
  * <pre class="code">&lt;property name="jndiEnvironment"&gt;
- * 	 &lt;props>
+ * 	 &lt;props&gt;
  *		 &lt;prop key="java.naming.factory.initial"&gt;com.sun.jndi.cosnaming.CNCtxFactory&lt;/prop&gt;
  *		 &lt;prop key="java.naming.provider.url"&gt;iiop://localhost:1050&lt;/prop&gt;
  *	 &lt;/props&gt;

spring-context/src/main/java/org/springframework/remoting/rmi/JndiRmiServiceExporter.java

@@ -52,7 +52,7 @@ import org.springframework.util.ReflectionUtils;
  * For example:
  *
  * <pre class="code">&lt;property name="jndiEnvironment"&gt;
- * 	 &lt;props>
+ * 	 &lt;props&gt;
  *		 &lt;prop key="java.naming.factory.initial"&gt;com.sun.jndi.cosnaming.CNCtxFactory&lt;/prop&gt;
  *		 &lt;prop key="java.naming.provider.url"&gt;iiop://localhost:1050&lt;/prop&gt;
  *	 &lt;/props&gt;

spring-context/src/main/java/org/springframework/ui/context/support/ResourceBundleThemeSource.java

@@ -87,7 +87,7 @@ public class ResourceBundleThemeSource implements HierarchicalThemeSource, BeanC
 	/**
 	 * Set the prefix that gets applied to the ResourceBundle basenames,
 	 * i.e. the theme names.
-	 * E.g.: basenamePrefix="test.", themeName="theme" -> basename="test.theme".
+	 * E.g.: basenamePrefix="test.", themeName="theme" → basename="test.theme".
 	 * <p>Note that ResourceBundle names are effectively classpath locations: As a
 	 * consequence, the JDK's standard ResourceBundle treats dots as package separators.
 	 * This means that "test.theme" is effectively equivalent to "test/theme",

spring-context/src/main/java/org/springframework/validation/Errors.java

@@ -83,8 +83,8 @@ public interface Errors {
 	 * {@code pushNestedPath(String)} call.
 	 * <p>Using the nested path stack allows to set temporary nested paths
 	 * for subobjects without having to worry about a temporary path holder.
-	 * <p>For example: current path "spouse.", pushNestedPath("child") ->
-	 * result path "spouse.child."; popNestedPath() -> "spouse." again.
+	 * <p>For example: current path "spouse.", pushNestedPath("child") &rarr;
+	 * result path "spouse.child."; popNestedPath() &rarr; "spouse." again.
 	 * @param subPath the sub path to push onto the nested path stack
 	 * @see #popNestedPath
 	 */