Drop support for configuring PropertiesLauncher with application.properties

Closes gh-8464

spring-boot-docs/src/main/asciidoc/appendix-executable-jar-format.adoc

@@ -201,12 +201,6 @@ the appropriate launcher:
 properties (System properties, environment variables, manifest entries or
 `loader.properties`).
 
-NOTE: `PropertiesLauncher` supports loading properties from
-`loader.properties` and also (for historic reasons)
-`application.properties`. We recommend using
-`loader.properties` exclusively, as support for
-`application.properties` is deprecated and may be removed in the future.
-
 |===
 |Key |Purpose
 

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/PropertiesLauncher.java

@@ -46,10 +46,10 @@ import org.springframework.boot.loader.util.SystemPropertyUtils;
  * well-behaved OS-level services than a model based on executable jars.
  * <p>
  * Looks in various places for a properties file to extract loader settings, defaulting to
- * {@code application.properties} either on the current classpath or in the current
- * working directory. The name of the properties file can be changed by setting a System
- * property {@code loader.config.name} (e.g. {@code -Dloader.config.name=foo} will look
- * for {@code foo.properties}. If that file doesn't exist then tries
+ * {@code loader.properties} either on the current classpath or in the current working
+ * directory. The name of the properties file can be changed by setting a System property
+ * {@code loader.config.name} (e.g. {@code -Dloader.config.name=foo} will look for
+ * {@code foo.properties}. If that file doesn't exist then tries
  * {@code loader.config.location} (with allowed prefixes {@code classpath:} and
  * {@code file:} or any valid URL). Once that file is located turns it into Properties and
  * extracts optional values (which can also be provided overridden as System properties in
@@ -156,8 +156,7 @@ public class PropertiesLauncher extends Launcher {
 			configs.add(getProperty(CONFIG_LOCATION));
 		}
 		else {
-			String[] names = getPropertyWithDefault(CONFIG_NAME, "loader,application")
-					.split(",");
+			String[] names = getPropertyWithDefault(CONFIG_NAME, "loader").split(",");
 			for (String name : names) {
 				configs.add("file:" + getHomeDirectory() + "/" + name + ".properties");
 				configs.add("classpath:" + name + ".properties");
@@ -175,10 +174,6 @@ public class PropertiesLauncher extends Launcher {
 					resource.close();
 				}
 				for (Object key : Collections.list(this.properties.propertyNames())) {
-					if (config.endsWith("application.properties")
-							&& ((String) key).startsWith("loader.")) {
-						warn("Use of application.properties for PropertiesLauncher is deprecated");
-					}
 					String text = this.properties.getProperty((String) key);
 					String value = SystemPropertyUtils
 							.resolvePlaceholders(this.properties, text);
@@ -608,19 +603,10 @@ public class PropertiesLauncher extends Launcher {
 
 	private void debug(String message) {
 		if (Boolean.getBoolean(DEBUG)) {
-			log(message);
+			System.out.println(message);
 		}
 	}
 
-	private void warn(String message) {
-		log("WARNING: " + message);
-	}
-
-	private void log(String message) {
-		// We shouldn't use java.util.logging because of classpath issues
-		System.out.println(message);
-	}
-
 	/**
 	 * Convenience class for finding nested archives that have a prefix in their file path
 	 * (e.g. "lib/").