Bump main to 18.11.0 after vs18.10 snap - #14564

Phase 3 of the 18.10 release checklist. Opens main for 18.11 now that vs18.10 has been cut.

⏸️ Do not merge yet. This must land only after the Phase 2 maestro-configuration PR (63340) is merged, so main's default channel is already VS 18.11 when the first bumped build publishes. Otherwise that build lands on the stale VS 18.10 channel.

Changes Made

3.1 — VersionPrefix 18.10.0 → 18.11.0.

3.2 — PackageValidationBaselineVersion → 18.10.0-1.26378.2. The latest Microsoft.Build package that is both published on the public dotnet-tools feed and built from a commit reachable from vs18.10 (official build 9434 run 20260728.2, sha bf5e076cdd).

3.3 — OptProfBaselineDrop refreshed to OptimizationData/DotNet-msbuild-Trusted/main/20260724.1/14773990/1, so the next vs* branch cut from main inherits valid OptProf data and its first official build succeeds without the manual Phase 4.2 rerun.

3.4 — Regenerated CompatibilitySuppressions.xml (4 files). See below.

Why the suppression files shrank

Moving the ApiCompat baseline from 18.9 to 18.10 made the baseline-specific CP0001 / CP0008 / PKV006 suppressions unnecessary, and the build treats unnecessary suppressions as an error. Those entries covered netstandard2.0 polyfill types (System.Index, System.Range, IsExternalInit, the System.Runtime.CompilerServices attributes, …) that differed against the 18.9 baseline but no longer differ against 18.10.

Only those entries were removed. The unrelated PKV004 framework suppressions are untouched — e.g. src/Framework still carries all 12 of them, and src/Tasks all 23.

Testing

Regenerated with build.cmd -pack /p:ApiCompatGenerateSuppressionFile=true, then verified with a clean pack run (no generate flag): 0 errors, 0 warnings.

Because a generation run does not prove validation passes, I also ran a control: git stash of these changes + clean artifacts/packages + build.cmd -pack on unmodified main → 0 errors. That confirms the suppression churn is attributable to the baseline change rather than a pre-existing failure, and that nothing is being masked.

⚠️ Follow-up: scripts/Get-PackageValidationBaseline.ps1 is stale

The Phase 3.2 helper cannot produce a correct answer any more and threw No eligible candidate builds found for 18.10. It hardcodes the old package format:

return "$ThisReleaseVersion.0-preview-$shortDate-$revStr"

but PreReleaseVersionLabel in eng/Versions.props is now 1, so official builds publish as 18.10.0-1.26378.2, not 18.10.0-preview-26378-02. The feed confirms the switchover — the last 18.10.0-preview-* was published 2026-07-10, everything since is 18.10.0-1.*.

The script is also stricter than the documented rule: it only accepts a build at the exact merge-base commit, but a branch cut minutes after a commit has no completed build at that sha yet. The checklist rule is "reachable from vs<THIS>", which admits any ancestor.

documentation/release-checklist.md and .github/skills/release/SKILL.md both document the -preview-NNNNN-NN format and need the same correction. Not fixed here to keep this PR to the branding change — happy to do it in a follow-up.