Pin GitHub action digest with semver (#6724)

Instead of only showing the major release, it adds full semver to the
comment of the GitHub action digest.

Example change:
491e6e8f69

renovate.json

@@ -1,7 +1,8 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": [
-    "config:best-practices"
+    "config:best-practices",
+    "helpers:pinGitHubActionDigestsToSemver"
   ],
   "ignorePaths": [],
   "labels": ["Skip Changelog", "dependencies"],