This commit updates the version parsing in make-release-checks.sh to use
sed instead of grep. The motivation for this is that currently when
running this script on macos it errors:
```console
$ ./scripts/make-release-checks.sh --dry-run
grep: invalid option -- P
usage: grep [-abcdDEFGHhIiJLlMmnOopqRSsUVvwXxZz] [-A num] [-B num] [-C[num]]
[-e pattern] [-f file] [--binary-files=value] [--color=when]
[--context[=num]] [--directories=action] [--label] [--line-buffered]
[--null] [pattern] [file ...]
```
With the changes in this commit it is possible to run this without
failure.