Correct plugin name of ExtendedWhitespaceThrowablePatternConverter

Previously, it had the same plugin name as
WhitespaceThrowablePatternConverter which meant that only one of the
two plugins was available and the other’s converter keys didn’t work.

Closes gh-4337

spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ExtendedWhitespaceThrowablePatternConverter.java

@@ -31,7 +31,7 @@ import org.apache.logging.log4j.core.pattern.ThrowablePatternConverter;
  * @author Phillip Webb
  * @since 1.3.0
  */
-@Plugin(name = "WhitespaceThrowablePatternConverter", category = PatternConverter.CATEGORY)
+@Plugin(name = "ExtendedWhitespaceThrowablePatternConverter", category = PatternConverter.CATEGORY)
 @ConverterKeys({ "xwEx", "xwThrowable", "xwException" })
 public final class ExtendedWhitespaceThrowablePatternConverter
 		extends ThrowablePatternConverter {