Disallow updating regexes that have set number 0 and type HOST_REGEXP

[srijeet0406]

What does this PR (Pull Request) do?

• This PR fixes Cannot update any regex in Traffic Ops/ Traffic Portal #5296 and potentialy Traffic Ops Does Not Update DNSSEC Keys on Delivery Service Update #2805

Which Traffic Control components are affected by this PR?

• CDN in a Box
• Traffic Ops

What is the best way to verify this PR?

Make sure all the unit/ API tests pass
Go to Traffic Portal
Try modifying a regex for any DS
This should be possible as long as the following two conditions are met:
1.) The regex in question does not have set number 0 and type HOST_REGEXP
2.) No other regex exists with the set number that you're trying to update the current regex to

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

• master

The following criteria are ALL met by this PR

• This PR includes tests
• I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md
• This PR includes any and all required license headers
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[mitchell852]

fyi the text in the description needs to look like this:

This PR fixes #2805 and fixes #5296

to autoclose both tickets.

[mitchell852]

I found a bug. I was able to change the regex type of the position 0 regex from HOST_REGEXP to:

[srijeet0406]

@mitchell852 Fixed in the latest commit

[mitchell852]

LGTM!