Fix links in Javadoc and reference docs

- Fix broken links (by using a new URL, an alternative URL, or a
  Wayback Machine link)

- Use HTTPS where possible

- Remove https://issuetracker.springsource.com/browse/EBR-349: this
  link is dead and is also mentioned in
  https://jira.spring.io/browse/SPR-8093

- Clean up nohttp allowlist.lines

Closes gh-28876

spring-test/src/main/java/org/springframework/test/web/client/match/ContentRequestMatchers.java

@@ -255,7 +255,7 @@ public class ContentRequestMatchers {
 	 * two are "similar" - i.e. they contain the same elements and attributes
 	 * regardless of order.
 	 * <p>Use of this matcher assumes the
-	 * <a href="http://xmlunit.sourceforge.net/">XMLUnit</a> library is available.
+	 * <a href="https://www.xmlunit.org/">XMLUnit</a> library is available.
 	 * @param expectedXmlContent the expected XML content
 	 */
 	public RequestMatcher xml(String expectedXmlContent) {

spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/MockMvcWebConnection.java

@@ -40,7 +40,7 @@ import org.springframework.util.Assert;
 /**
  * {@code MockMvcWebConnection} enables {@link MockMvc} to transform a
  * {@link WebRequest} into a {@link WebResponse}.
- * <p>This is the core integration with <a href="http://htmlunit.sourceforge.net/">HtmlUnit</a>.
+ * <p>This is the core integration with <a href="https://htmlunit.sourceforge.io/">HtmlUnit</a>.
  * <p>Example usage can be seen below.
  *
  * <pre class="code">

spring-test/src/main/java/org/springframework/test/web/servlet/result/ContentResultMatchers.java

@@ -160,7 +160,7 @@ public class ContentResultMatchers {
 	 * are "similar" - i.e. they contain the same elements and attributes
 	 * regardless of order.
 	 * <p>Use of this matcher requires the <a
-	 * href="http://xmlunit.sourceforge.net/">XMLUnit</a> library.
+	 * href="https://www.xmlunit.org/">XMLUnit</a> library.
 	 * @param xmlContent the expected XML content
 	 * @see MockMvcResultMatchers#xpath(String, Object...)
 	 * @see MockMvcResultMatchers#xpath(String, Map, Object...)