Keep pre-commit hooks up to date using Dependabot

- Add `pre-commit` as an ecosystem for Dependabot version updates,
  now that it is available as a beta ecosystem. Enable beta
  ecosystems to allow this.

- Group the updates and use a monthly cadence to avoid getting
  swamped by frequent automated PRs.

- Use a 7-day cooldown period rather than taking new versions
  immediately when they are released. (This may also be of value to
  developers who use `pre-commit` locally.)

- Use a less busy style (less unnecessary quoting) than was being
  used in `dependabot.yml` before, since this new stanza is more
  elaborate than before. Apply that style to the existing stanzas
  for consistency.

Author: EliahKagan

.github/dependabot.yml

@@ -1,11 +1,22 @@
 version: 2
+enable-beta-ecosystems: true
 updates:
-- package-ecosystem: "github-actions"
+- package-ecosystem: github-actions
   directory: "/"
   schedule:
-    interval: "weekly"
+    interval: weekly
 
-- package-ecosystem: "gitsubmodule"
+- package-ecosystem: gitsubmodule
   directory: "/"
   schedule:
-    interval: "weekly"
+    interval: weekly
+
+- package-ecosystem: pre-commit
+  directory: "/"
+  schedule:
+    interval: monthly
+  groups:
+    pre-commit:
+      patterns: ["*"]
+  cooldown:
+    default-days: 7