[MNG-7797] Return BRIEF mode (#1132)

Make BRIEF a legal value again.

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https://issues.apache.org/jira/browse/MNG-7797

maven-core/src/main/java/org/apache/maven/plugin/internal/DefaultPluginValidationManager.java

@@ -58,6 +58,7 @@ private enum ValidationReportLevel {
         NONE, // mute validation completely (validation issue collection still happens, it is just not reported!)
         INLINE, // inline, each "internal" problem one line next to mojo invocation
         SUMMARY, // at end, list of plugin GAVs along with "internal" issues
+        BRIEF, // synonym to SUMMARY
         VERBOSE // at end, list of plugin GAVs along with detailed report of ANY validation issues
     }