Use tilde for rc tag versioning

Use tilde for rc tag versioning

tilde sorts lower in the version comparison spec:
https://uapi-group.org/specifications/specs/version_format_specification/

➜  systemd git:(strip) systemd-analyze compare-versions 249\~rc1 249
249\~rc1 < 249
➜  systemd git:(strip) systemd-analyze compare-versions 249-rc1 249
249-rc1 > 249

Also update tools/meson-vcs-tag.sh to use carets instead of hyphens
for the git part of the version as carets are allowed to be part of
a version by pacman while hyphens are not and both sort higher than
a version without the git part.