fedora-coreos-config: bump to 21b18cf

```
$ git shortlog a08de30..21b18cf
Benjamin Gilbert (1):
      overrides: fast-track coreos-installer 0.10.1-1.fc34 for CVE-2021-20319

Dusty Mabe (5):
      tests: fix mulipathd.socket test
      move multipathd.socket workaround from overlay to manifest postprocess
      move multipathd.service workaround from overlay to manifest postprocess
      add shared-workarounds.yaml manifest
      tests: break out multipath fix tests from misc-ro
```