Rename plugins to file hooks

What does this MR do?

Renames plugins feature to file hooks. Per #196051 (closed).

• Rename all code and specs
• Rename rake tasks
• Rename the documentation
• Sidekiq queue

Not renamed:

• plugins directory. Because all existing plugins will stay in old directory. Keep till major release? Support both plugins and file_hooks directories at the same time? Follow-up issue => #196846 (closed)
• log file. Keep till major release? Follow-up issue => #196846 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team