Only run Last-Server-In-Cache Group validation when Cache Group has changed

[zrhoffman]

What does this PR (Pull Request) do?

• This PR fixes Last-Server-In-Cache Group validation runs when Cache Group has not changed #5165 OR is not related to any Issue
• This PR makes Last-Server-In-Cache Group validation from Disallow deleting or moving the last server in a Cache Group used by a topology #5155 only run if the Cache Group has changed

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Run API tests or

1. Make a Cache Group
2. Make a Server
3. Add the Server to the Cache Group
4. Make a Topology
5. Add the Server to the Topology
6. Update the Server without changing anything

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

• master (bda1a51)

The following criteria are ALL met by this PR

• This PR includes tests
• Intended behavior is unchanged, no documentation necessary
• Bugfix for a feature not yet in a release, no CHANGELOG.md entry necessary
• 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)

Additional Information

[rawlinp]

Checks passed, code looks sound.