Bump systemd/mkosi from 18 to 21

Bumps [systemd/mkosi](https://github.com/systemd/mkosi) from 18 to 21.
- [Release notes](https://github.com/systemd/mkosi/releases)
- [Changelog](https://github.com/systemd/mkosi/blob/main/NEWS.md)
- [Commits](systemd/mkosi@v18...v21)

---
updated-dependencies:
- dependency-name: systemd/mkosi
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/sysexts.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
       - name: Setup mkosi
-        uses: systemd/mkosi@v18
+        uses: systemd/mkosi@v21
       - name: Build base image
         run: sudo mkosi --architecture=${{ matrix.arch }}
         working-directory: ./sysexts/mkosi/debian-testing-base