[vs18.9] Disable unavailable OptProf data

[AR-May]

Follows #14903 (vs18.6), #14902 (vs18.0) and #14909 (vs16.11).

Changes Made

• Changed enableOptProf to default to false.
• Kept the existing behavior that sets SkipApplyOptimizationData=true when OptProf collection is disabled, avoiding the unavailable-data restore.
• Gated the MicroBuildOutputs artifact publish on enableOptProf, since it is only produced when the OptProf bootstrapper runs.
• Moved the MicroBuild/Output staging path into the enableOptProf branch of the artifact-layout check for the same reason.
• Bumped VersionPrefix to 18.9.15.

Notes

• Pipeline configuration only; no product code changes.

[Copilot]

🟢 Approval recommended

The pipeline-only changes are consistent with existing enableOptProf gating patterns in the same templates and align with the stated goal of preventing OptProf-related build failures on this servicing branch.

Pull request overview

Disables OptProf data collection by default on vs18.9 to avoid failures caused by unavailable optimization data / assembly version mismatches during the OptProf collection run, while keeping the existing SkipApplyOptimizationData=true behavior when collection is disabled.

Changes:

• Set enableOptProf default to false for this branch and document the rationale in the pipeline parameters.
• Gate publishing of the MicroBuildOutputs artifact on enableOptProf, since it’s only produced when the OptProf bootstrapper runs.
• Move the MicroBuild/Output staging-path check into the enableOptProf branch of the artifact layout validation; bump VersionPrefix to 18.9.15.

File summaries

File	Description
eng/Versions.props	Bumps VersionPrefix to 18.9.15 for the servicing branch.
azure-pipelines/.vsts-dotnet-build-jobs.yml	Conditions MicroBuildOutputs publication and OptProf-only staging-path validation on enableOptProf.
.vsts-dotnet.yml	Defaults enableOptProf to false and documents why OptProf is disabled on this branch.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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