docs: update references to ghcr.io/renovatebot/renovate to v39.57.4

| datasource | package                      | from    | to      |
| ---------- | ---------------------------- | ------- | ------- |
| docker     | ghcr.io/renovatebot/renovate | 39.28.0 | 39.57.4 |

README.md

@@ -222,7 +222,7 @@ The Renovate version to use.
 If omitted the action will use the [`default version`](./action.yml#L28) Docker tag.
 Check [the available tags on Docker Hub](https://hub.docker.com/r/renovate/renovate/tags).
 
-This sample will use `ghcr.io/renovatebot/renovate:39.28.0` image.
+This sample will use `ghcr.io/renovatebot/renovate:39.57.4` image.
 
 ```yml
 ....
@@ -235,7 +235,7 @@ jobs:
       - name: Self-hosted Renovate
         uses: renovatebot/github-action@v41.0.5
         with:
-          renovate-version: 39.28.0
+          renovate-version: 39.57.4
           token: ${{ secrets.RENOVATE_TOKEN }}
 ```
 
@@ -514,7 +514,7 @@ jobs:
         with:
           configurationFile: renovate.json5
           token: ${{ secrets.RENOVATE_TOKEN }}
-          renovate-version: 39.28.0
+          renovate-version: 39.57.4
         env:
           # This enables the cache -- if this is set, it's not necessary to add it to renovate.json.
           RENOVATE_REPOSITORY_CACHE: ${{ github.event.inputs.repoCache || 'enabled' }}