feat: add renovate regex for gha workflows version variable (#16)

This should be able to match the following strings:
```yaml
jobs:
  lint:
    steps:
      - uses: golangci/golangci-lint-action@v3
        with:
          version: v1.55.1 # renovate: datasource=github-releases depName=golangci/golangci-lint
```

renovate/default.json

@@ -6,5 +6,13 @@
     ":semanticCommitScopeDisabled",
     ":semanticCommitTypeAll(deps)",
     ":enablePreCommit"
+  ],
+  "regexManagers": [
+    {
+      "fileMatch": ["^\\.github\\/(?:workflows|actions)\\/.+\\.ya?ml$"],
+      "matchStrings": [
+        "^\\s+(?:[a-zA-Z-_]+)?(?:version|VERSION): (?<currentValue>.+?) # renovate: datasource=(?<datasource>[a-z-]+?) depName=(?<depName>.+?)(?: lookupName=(?<lookupName>.+?))?(?: versioning=(?<versioning>[a-z-]+?))?$"
+      ]
+    }
   ]
 }