Fixed DSR difference for DS required capability

[rimashah25]

DSR difference (current value) for DS capability will only show when changes have been made to it.

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Which Traffic Control components are affected by this PR?

• Traffic Portal

What is the best way to verify this PR?

To reproduce the error,

1. For an HTTP/DNS DS, update any value (not required capability) and submit the DSR
2. In DSR, you will notice the diff for required capability as follows:

3. Now run this branch, and check for the same error
4. You shouldn't see the error any more.
5. Also, ensure that when you do update required capability, DSR shows is correctly.

If this is a bugfix, which Traffic Control versions contained the bug?

7.0.1

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)