Upgrade to JGit 7

[jonesbusy]

Fix #107

JGit doesn't providing signing with SSH, but at least it doesn't fail because the constant doesn't exists

No breaking changes expected. JGit 7 require now JDK 17 but our tool is already built with Java 17

I would suggest we open a small improvement to setup GPG signing if requested by the user

And my .gitconfig

[user]
	signingkey = /home/valentin/.ssh/id_ed25519
[gpg]
	format = ssh
[commit]
	gpgsign = true

Testing done

Tested by opening this pr jenkinsci/asm-api-plugin#39

Some tests are already prepared for the git plugin

jenkinsci/git-client-plugin#1170
jenkinsci/git-client-plugin#1172

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[gounthar]

Thanks!