Make sure custom finalName is ignored

A read-only plugin parameter can still be set by the user which leads
to an invalid repackaged archive. This commit actually ignores the field
and uses the standard attribute instead.

Closes spring-projectsgh-16202

Author: snicoll

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

@@ -263,7 +263,8 @@ private File getTargetFile() {
 		if (!this.outputDirectory.exists()) {
 			this.outputDirectory.mkdirs();
 		}
-		return new File(this.outputDirectory, this.finalName + classifier + "."
+		String finalName = this.project.getBuild().getFinalName();
+		return new File(this.outputDirectory, finalName + classifier + "."
 				+ this.project.getArtifact().getArtifactHandler().getExtension());
 	}