Group minor/patch version Ruby Dependabot updates into one PR (#1479)

This aims to reduces Dependabot PR noise by enabling Dependabot's
new semver version level grouping feature:
https://github.blog/changelog/2023-08-17-grouped-version-updates-by-semantic-version-level-for-dependabot/

It's only enabled for Ruby dependencies, since for this repo they are
typically the most noisy, plus are development-only dependencies
that typically don't need any compatibility changes so bundling them
together won't make debugging any harder.

GUS-W-13990911.

.github/dependabot.yml

@@ -8,6 +8,11 @@ updates:
       - "dependencies"
       - "ruby"
       - "skip changelog"
+    groups:
+      ruby-dependencies:
+        update-types:
+          - "minor"
+          - "patch"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: