ci(dependabot): check for action updates monthly

When workflows referred to actions by major version tag, such as
`actions/checkout@v7`, Dependabot proposed a change only for a new
major version, which is infrequent. Now that every action is pinned to
a full commit hash with a `# vX.Y.Z` comment, Dependabot proposes a
change for every new release, including minor and patch releases. With
a weekly schedule, that would produce many more pull requests than
before, most of them for small changes.

Check for GitHub Actions updates monthly instead, as is already done
for `pre-commit` hooks. Each scheduled run then proposes, for each
action, its newest release that is at least 7 days old, so intervening
releases of the same action are skipped rather than proposed one by
one. Dependabot security updates, where enabled, do not follow this
schedule.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: EliahKagan

.github/dependabot.yml

@@ -3,7 +3,7 @@ updates:
 - package-ecosystem: github-actions
   directory: "/"
   schedule:
-    interval: weekly
+    interval: monthly
   cooldown:
     default-days: 7