Restrict junit updates on maintenance branches (#2410)

Author: acoburn

.github/dependabot.yml

@@ -34,6 +34,12 @@ updates:
       - dependency-name: "com.google.protobuf:protobuf-java"
         update-types:
           - "version-update:semver-major"
+      - dependency-name: "org.junit.jupiter:junit-*"
+        update-types:
+          - "version-update:semver-major"
+      - dependency-name: "org.junit:junit-*"
+        update-types:
+          - "version-update:semver-major"
     labels:
       - "backport"
       - "1.2"
@@ -103,6 +109,12 @@ updates:
       - dependency-name: "com.google.protobuf:protobuf-java"
         update-types:
           - "version-update:semver-major"
+      - dependency-name: "org.junit.jupiter:junit-*"
+        update-types:
+          - "version-update:semver-major"
+      - dependency-name: "org.junit:junit-*"
+        update-types:
+          - "version-update:semver-major"
     labels:
       - "backport"
       - "1.3"