release-24.1: skip: remove skip.UnderStressRace{,WithIssue}

[rickystewart]

Backport 1/1 commits from #123910.

/cc @cockroachdb/release

---

... and replace with UnderRace.

We are eliminating our use of https://github.com/cockroachdb/stress in cockroachdb/cockroach CI. In our nightlies on master and release-24.1, we are already not using stress (in favor of remote execution). It's still used in some nightlies (like the Pebble nightly) and Bazel Extended CI, though these uses will presumably be reduced as well.

Meanwhile, the difference between these two functions is not clear and it's not always obvious which you should pick. Prior to the remote execution-based "stress" nightly, either would work. With remote execution, only skip.UnderRace works (as this is not "stress" as far as skip is concerned). This is confusing.

Even before we were using remote execution, we were running the nightly stressrace with only one concurrent test run, so this was not any more "stressful" than not using stress, further confusing which skip function was appropriate to use.

We migrate instead to simply having one function: skip.UnderRace.

Epic: none

Release note: None

Release justification: Test-only code changes

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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  here).
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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Also, please add a brief release justification to the body of your PR to justify this
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[cockroach-teamcity]

This change is