Fixed stress test pipeline and other issues.

[ManickaP]

Addresses multiple issues with HTTP stress:

• HTTP 1.1 won't run at all if HTTP 2.0 failed
  • Adding conditions that ensure that the step won't run only if build failed, not the previous stress run step.
• Pipeline naming inconsistencies
• Parameter nullability inspired by [release/5.0] Fix OS version check and testhost copying in stress tests #41321 (review)
  • Leaving [DisallowNull] in since that's the way it is in the interface: https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IEqualityComparer.cs#L14
• The final report will log if the failed run has been intentionally cancelled, to improve investigation of existing H/2 failures.

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[ManickaP]

Runtime test failure: #41078; build failure: #36860.

[alnikola]

LGTM