deps: Bump npm to v7 (#564)

And updated engines field

cf. [npm 7 is now generally available! - The GitHub Blog](https://github.blog/2021-02-02-npm-7-is-now-generally-available/)

package.json

@@ -4,8 +4,8 @@
   "description": "GitHub Actions for GitHub Pages",
   "main": "lib/index.js",
   "engines": {
-    "node": ">=12.16.3",
-    "npm": ">=6.14.4"
+    "node": ">=12.20.1",
+    "npm": ">=7.5.2"
   },
   "scripts": {
     "all": "npm run format && npm run lint && npm test",