major: Use git cli instead of grgit

[ajoberstar]

grgit is not really necessary for this plugin, decoupling it will improve a lot of things.

The expectation is for behavior equivalence, but I expect there will be some quirks from switching so this is considered a major version.

[ciscoo]

I think this change will help in resolving an issue I'm having internally at my company. Currently, publishing our documentation fails with an unhelpful error:

* What went wrong:
Execution failed for task ':documentation:gitPublishPush'.
  > org.ajoberstar.grgit.PushException: Failed to push: refs/heads/gh-pages to refs/heads/gh-pages (pre-receive hook declined)

JGit doesn't support several Git hooks and currently, any error raised is lost. Enabling debug logs for JGit also doesn't help either:

[DEBUG] [org.eclipse.jgit.transport.PacketLineIn] git< unpack ok
[DEBUG] [org.eclipse.jgit.transport.PacketLineIn] git< ng refs/heads/gh-pages pre-receive hook declined
[DEBUG] [org.eclipse.jgit.transport.PacketLineIn] git< 0000

We use owasp/sedated which is a pre-receive hook. It is a bit of a pain to figure out what is being blocked when the only log from JGit doesn't provide any information.

By using the git CLI directly, this should surface the error and anything else really which should help.