[MCOMPILER-576] Deprecate parameter "compilerVersion"

This is no longer evaluated in plexus-compiler-javac
(codehaus-plexus/plexus-compiler#356).
It was never used by any other plexus compilers in the past.

src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java

@@ -236,7 +236,10 @@ public abstract class AbstractCompilerMojo extends AbstractMojo {
 
     /**
      * Version of the compiler to use, ex. "1.3", "1.5", if {@link #fork} is set to <code>true</code>.
+     * @deprecated This parameter is no longer evaluated by the underlying compilers, instead the actual
+     * version of the {@code javac} binary is automatically retrieved.
      */
+    @Deprecated
     @Parameter(property = "maven.compiler.compilerVersion")
     private String compilerVersion;