Switch bundler ecosystem to monthly updates

Since we have auto merges as well, no need for such frequently update checks. If an important release is available that we'd like to include and test, then we can easily run Dependabot manually to pick it up. Otherwise, we go many months without explicit interaction needed.

.github/dependabot.yml

@@ -7,15 +7,15 @@ updates:
   - package-ecosystem: bundler
     directory: /
     schedule:
-      interval: daily
+      interval: monthly
     versioning-strategy: lockfile-only
   - package-ecosystem: bundler
     directory: /gemfiles/rails_61
     schedule:
-      interval: daily
+      interval: monthly
     versioning-strategy: lockfile-only
   - package-ecosystem: bundler
     directory: /gemfiles/rails_70
     schedule:
-      interval: daily
+      interval: monthly
     versioning-strategy: lockfile-only