ci: use the same version of p4 on both Linux and macOS

ci: use the same version of p4 on both Linux and macOS

There would be a segmentation fault when running p4 v16.2 on ubuntu
22.04 which is the latest version of ubuntu runner image for github
actions.

By checking each version from [1], p4d version 21.1 and above can work
properly on ubuntu 22.04. But version 22.x will break some p4 test
cases. So p4 version 21.x is exactly the version we can use.

With this update, the versions of p4 for Linux and macOS happen to be
the same. So we can add the version number directly into the "P4WHENCE"
variable, and reuse it in p4 installation for macOS.

By removing the "LINUX_P4_VERSION" variable from "ci/lib.sh", the
comment left above has nothing to do with p4, but still applies to
git-lfs. Since we have a fixed version of git-lfs installed on Linux,
we may have a different version on macOS.

[1]: https://cdist2.perforce.com/perforce/

Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>