Bump docker/login-action from 3 to 4

Bumps [docker/login-action](https://github.com/docker/login-action) from 3 to 4.
- [Release notes](https://github.com/docker/login-action/releases)
- [Commits](docker/login-action@v3...v4)

---
updated-dependencies:
- dependency-name: docker/login-action
  dependency-version: '4'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/mainline-prepare.yml

@@ -21,7 +21,7 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Log in to GHCR
-        uses: docker/login-action@v3
+        uses: docker/login-action@v4
         with:
           registry: ghcr.io
           username: ${{ github.actor }}