Deprecate & Rewrite DELETE to_extension

[mhoppa]

What does this PR (Pull Request) do?

• This PR fixes Rewrite /to_extensions/{{id}}/delete to Go #3845

Deprecates old route of POST /to_extension/:id/delete with notices in perl response and in docs and adds new route DELETE /to_extension/:id

Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Control Client
• Traffic Ops

What is the best way to verify this PR?

Make a request to POST /to_extension/:id/delete make sure the deprecation notice is returned, make another request to DELETE /to_extension/:id and ensure that deletes a to_extension, run the API tests.

If this is a bug fix, what versions of Traffic Control are affected?

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

Dependent on #4327

[ocket8888]

Manual testing shows the new endpoint works identically to the old one, and matches the docs which compile without warnings or errors and look accurate and well-formed, the old endpoint displays the deprecation warning in all applicable scenarios, all unit and client/api integration tests are passing.