Deprecate GET tenants/:id

[mhoppa]

What does this PR (Pull Request) do?

• This PR is not related to any Issue

Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Ops

What is the best way to verify this PR?

Run traffic ops and make sure you can not hit the endpoint in API 2.0 and when performing a GET on it in 1.x you receive a deprecation message back in the returned alerts.

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

[shamrickus]

LGTM

• No go fmt issues
• TO unit/integration tests pass
• No doc issues
• 2.0 endpoint is gone, 1.1 endpoint has a deprecation alert
• All references in ORT/clients updated

[shamrickus]

There is a merge conflict though