Remove opam 2.1 support from the release script

[kit-ty-kate]

Simplifies the release script and makes it less likely to have errors
Related to #6082 and #6083

Backported to 2.2 in #6085

[rjbou]

Shouldn't we also add in the release readme the fact that now you need to be in the branch you want to release (and not just fetch the tag)

[kit-ty-kate]

done

[rjbou]

Thanks!