Add Maven compiler problem matcher for javac diagnostics

The javac problem matcher only recognized javac's native diagnostic
format (File.java:12: warning: msg), which works for plain javac and
Gradle but not Maven. The maven-compiler-plugin reformats diagnostics
to [WARNING] /path/File.java:[12,5] msg, so Maven builds produced zero
annotations.

Add a new maven-javac matcher owner that recognizes the Maven format,
capturing severity, file, line, column, and message. Maven builds now
annotate consistently with Gradle and plain javac.

Fixes: #1085

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

.github/java.json

@@ -21,6 +21,19 @@
                     "message": 4
                 }
             ]
+        },
+        {
+            "owner": "maven-javac",
+            "pattern": [
+                {
+                    "regexp": "^\\[(WARNING|ERROR)\\]\\s+(.+?\\.java):\\[(\\d+),(\\d+)\\]\\s+(.+)$",
+                    "severity": 1,
+                    "file": 2,
+                    "line": 3,
+                    "column": 4,
+                    "message": 5
+                }
+            ]
         }
     ]
 }