Update outdated images for Source Code docs

What does this MR do and why?

This MR updates screenshots in the following pages:

• Contains images from v12:

  • doc/administration/settings/push_event_activities_limit.md

• Contains images from v13:

  • doc/user/snippets.md
  • doc/topics/git/git_rebase.md
  • doc/user/project/repository/forking_workflow.md
  • doc/user/project/file_lock.md
  • doc/user/project/codeowners/index.md

Related issue

• https://gitlab.com/gitlab-org/technical-writing/-/issues/842+

Review app

• Push event activities limit and bulk push events
• Snippets
• Forks
• Code Owners