Usually tags can be downloaded using git fetch -t, but not for 2.21.1
The tag 2.21.1 does not belong to any branch, see: https://github.com/geoserver/geoserver/tree/2.21.1.
At least the actual commit of 2.21.1 can be fetched using:
git fetch origin 6a17a2ee2607c4b578d0d7aa242d999ffa2d9500
When checking it out you end up with a detached head.
Everything works fine with tag 2.21.0. Something wired happens with 2.21.1.