Tweak Dependabot version updates config.

Author: devopsix

.github/dependabot.yml

@@ -2,6 +2,9 @@ version: 2
 updates:
   - package-ecosystem: maven
     directory: '/'
+    ignore:
+      - dependency-name: 'com.icegreen:greenmail-junit5'
+        update-types: [ 'version-update:semver-major' ]
     schedule:
       interval: 'daily'
     open-pull-requests-limit: 100