Deprecate legacy disk paths

What does this MR do?

Log deprecation warnings when the legacy disk path is used, which we can use to determine if spots where missed.

The test environment won't, sadly, spawn these messages, as it uses disk access to setup and tear down the required state.

Example when triggered, here a testing support file just as it was easy to trigger it:

DEPRECATION WARNING: legacy_disk_path is deprecated and will be removed from GitLab 11.0. (called from repos_path at /Users/zegerjan/Development/gitlab-development-kit/gitlab/spec/support/test_env.rb:229)
DEPRECATION WARNING: legacy_disk_path is deprecated and will be removed from GitLab 11.0. (called from repos_path at /Users/zegerjan/Development/gitlab-development-kit/gitlab/spec/support/test_env.rb:229)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?