Deprecate /riak/stats

[mhoppa]

What does this PR (Pull Request) do?

• This PR fixes Rewrite /riak/stats to Go #3859

Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Ops

What is the best way to verify this PR?

Make a Get /riak/stats request and make sure a deprecation notice is returned. No docs changes as no documentation exists for endpoint and no reason to add it for deprecated routes.

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

[ocket8888]

Manually sent GET request for /riak/stats and saw the warning.