Use Git checker for all elementary repositories

[davidmhewitt]

The json checker for the Stylesheet introduced in #155 won't necessarily work for all releases, because we don't upload release tarballs in the expected way. See the empty assets array at https://api.github.com/repos/elementary/stylesheet/releases/latest

compared to the following for libportal:
https://api.github.com/repos/flatpak/libportal/releases/latest

We can use the git checker instead to bump the tag and commit hash when there are updates. So I've replaced the stylesheet checker and added it for the other elementary repositories. Testing locally notices the outdated granite:

OUTDATED: granite.git
 Has a new version:
  URL:       https://github.com/elementary/granite.git
  Commit:    f190e26e1c850b3ab078357f1dcedd6e8278f61f
  Tag:       7.5.0
  Branch:    None
  Version:   7.5.0
  Timestamp: None

INFO    src.main: Check finished with 0 error(s)

[danirabbit]

TIL there's a git checker. That's super useful!