[4.1.x] Make 4.1.x TM backwards compatible with pre-4.0 TO

[rawlinp]

What does this PR (Pull Request) do?

NOTE: this is not a true backport, but these changes were extracted from
similar changes made to TM in master.

• This PR fixes Traffic Monitor Cannot Be Upgraded Independently of Traffic Ops #5005

Which Traffic Control components are affected by this PR?

• Traffic Monitor

What is the best way to verify this PR?

With a pre-4.0 TO, upgrade TM to this version, verify that TM attempts to use TO API 2.0 before falling back to 1.3.

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

• 4.0
• 4.1

The following criteria are ALL met by this PR

• This PR is a pseudo-backport, but this change had no corresponding tests
• bugfix, no docs necessary
• This PR includes an update to CHANGELOG.md
• This PR includes any and all required license headers
• 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)

[rawlinp]

Merging this per the testing feedback from @smalenfant.