Fail fast when attempting to repackage a reproducible war

Maven's war plugin does not support reproducible builds, resulting in
the entries in the war file not being written in a consistent order
from build to build.

Closes gh-20176

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RepackageMojo.java

@@ -146,7 +146,7 @@ public class RepackageMojo extends AbstractPackagerMojo {
 	/**
 	 * Timestamp for reproducible output archive entries, either formatted as ISO 8601
 	 * (<code>yyyy-MM-dd'T'HH:mm:ssXXX</code>) or an {@code int} representing seconds
-	 * since the epoch.
+	 * since the epoch. Not supported with war packaging.
 	 * @since 2.3.0
 	 */
 	@Parameter(defaultValue = "${project.build.outputTimestamp}")