Rename plugin and hook features

During call with @sytses we decided to rename some of our features related to plugins and hooks. For better picture here is a table of all "hooks and plugins" features we have:

Feature name	Description	New name
Plugins	https://docs.gitlab.com/ee/administration/plugins.html same trigger as system hooks but execute files on filesystem, not able to modify UI or anything else you would expect of plugins	File hooks
Custom server-side Git hooks	https://docs.gitlab.com/ee/administration/custom_hooks.html	Server hooks
Git hooks	https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks is git hooks, client side, not made by GitLab
System hooks	https://docs.gitlab.com/ee/system_hooks/system_hooks.html work like web hook but above project level
Group hooks	#19292 (issue should be renamed)
Web hooks	https://docs.gitlab.com/ee/user/project/integrations/webhooks.html

TODO:

• Rename plugins to file hooks => !22979 (merged)
• Rename Custom server-side Git hooks to Server hooks => !23064 (merged)
• Rename issue to Group hooks => #19292