Make PlaceholderResolutionException extend from IllegalArgumentException

To smooth upgrade from 6.1.x, this commit makes sure that code that used
to catch an IAE to ignore a faulty placeholder resolution still works.

See gh-9628

Author: snicoll

Diff for: spring-core/src/main/java/org/springframework/util/PlaceholderResolutionException.java

@@ -31,7 +31,7 @@
  * @since 6.2
  */
 @SuppressWarnings("serial")
-public class PlaceholderResolutionException extends RuntimeException {
+public class PlaceholderResolutionException extends IllegalArgumentException {
 
 	private final String reason;