Adding 3.1.x branch to dependabot

Author: Ryan Baxter

.github/dependabot.yml

@@ -20,6 +20,17 @@ updates:
       - dependency-name: "*"
         update-types:
           - version-update:semver-major
+  - package-ecosystem: maven
+    directory: /
+    schedule:
+      interval: daily
+    target-branch: 3.1.x
+    ignore:
+      # only upgrade by minor or patch
+      - dependency-name: "*"
+        update-types:
+          - version-update:semver-major
+          - version-update:semver-minor
   - package-ecosystem: npm
     target-branch: docs-build
     directory: /