Add GCS cache backend with GitHub fallback

Port GCS support from danySam/gcs-cache. When gcs-bucket input is set,
cache to Google Cloud Storage via Application Default Credentials; else
fall back to GitHub's cache service transparently.

- New inputs: gcs-bucket, gcs-path-prefix (default: github-cache)
- New src/utils/gcsCache.ts wraps @actions/cache restore/save
- restoreImpl/saveImpl swap @actions/cache import for local wrapper
- Adds @google-cloud/storage ^7.16.0, bumps typescript to ^5.8.3
- Regenerated dist/ via npm run build

Skipped: upstream workflow removals, README/examples/RELEASES churn,
licensed/codeql config, GCS integration-test workflow.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "cache",
-  "version": "5.0.4",
+  "version": "4.2.3",
   "private": true,
   "description": "Cache dependencies and build outputs",
   "main": "dist/restore/index.js",
@@ -23,32 +23,30 @@
   "author": "GitHub",
   "license": "MIT",
   "dependencies": {
-    "@actions/cache": "^5.0.5",
-    "@actions/core": "^2.0.3",
-    "@actions/exec": "^2.0.0",
-    "@actions/io": "^2.0.0"
+    "@actions/cache": "^4.0.3",
+    "@actions/core": "^1.11.1",
+    "@actions/exec": "^1.1.1",
+    "@actions/io": "^1.1.3",
+    "@google-cloud/storage": "^7.16.0"
   },
   "devDependencies": {
-    "@types/jest": "^29.5.14",
+    "@types/jest": "^27.5.2",
     "@types/nock": "^11.1.0",
-    "@types/node": "^24.1.0",
-    "@typescript-eslint/eslint-plugin": "^7.2.0",
-    "@typescript-eslint/parser": "^7.2.0",
+    "@types/node": "^16.18.3",
+    "@typescript-eslint/eslint-plugin": "^5.45.0",
+    "@typescript-eslint/parser": "^5.45.0",
     "@vercel/ncc": "^0.38.3",
     "eslint": "^8.28.0",
-    "eslint-config-prettier": "^9.1.2",
-    "eslint-plugin-import": "^2.32.0",
-    "eslint-plugin-jest": "^27.9.0",
-    "eslint-plugin-prettier": "^5.5.3",
-    "eslint-plugin-simple-import-sort": "^12.1.1",
-    "jest": "^29.7.0",
-    "jest-circus": "^29.7.0",
+    "eslint-config-prettier": "^8.5.0",
+    "eslint-plugin-import": "^2.26.0",
+    "eslint-plugin-jest": "^26.9.0",
+    "eslint-plugin-prettier": "^4.2.1",
+    "eslint-plugin-simple-import-sort": "^7.0.0",
+    "jest": "^28.1.3",
+    "jest-circus": "^27.5.1",
     "nock": "^13.2.9",
-    "prettier": "^3.6.2",
-    "ts-jest": "^29.4.0",
+    "prettier": "^2.8.0",
+    "ts-jest": "^28.0.8",
     "typescript": "^5.8.3"
-  },
-  "engines": {
-    "node": ">=24"
   }
-}
+}