ci: let renovate manage github action versions

enabledManagers was limited to npm, so action versions never got update PRs
and drifted up to three majors behind — checkout and setup-node were still on
v4 while v7 was current.

The existing packageRules all match on `matchManagers: ["npm"]`, so action
updates fall through to the defaults: individual PRs, no automerge, reviewed
like any other major.

renovate.json

@@ -101,7 +101,7 @@
   "prHourlyLimit": 2,
   "prConcurrentLimit": 5,
   "updatePinnedDependencies": false,
-  "enabledManagers": ["npm"],
+  "enabledManagers": ["npm", "github-actions"],
   "ignoreDeps": [],
   "ignorePaths": ["**/node_modules/**", "**/dist/**", "**/coverage/**"]
 }