Upgrade to V4 Arcade publishing

[mmitche]

Upgrade to V4 Arcade publishing

This PR migrates dotnet/metadata-tools from Arcade V3 publishing to V4 publishing.

Changes

File	Change
eng/Publishing.props	PublishingVersion 3 → 4
azure-pipelines.yml	Removed enablePublishUsingPipelines, added publishingVersion: 4 and enablePublishing: true on Windows job, removed /p:DotNetPublishUsingPipelines=true from build args, publishingInfraVersion 3 → 4
azure-pipelines-pr.yml	Removed _PublishUsingPipelines variable, removed enablePublishUsingPipelines, removed /p:DotNetPublishUsingPipelines from build args, publishingInfraVersion 3 → 4

Validation tracking

• Official validation build: https://dev.azure.com/dnceng/internal/_build/results?buildId=2957932 (partiallySucceeded — Source-Build failed with pre-existing NU1510 error unrelated to V4 changes; Windows Release, Publish Assets, and Finalize all succeeded)
• Promotion build: https://dev.azure.com/dnceng/internal/_build/results?buildId=2957983 (partiallySucceeded with only expected V3 fallback download warnings)
• Asset comparison: baseline BAR 307222 (build 20260322.1) vs new BAR 311487 (build 20260422.6) — 5 assets each, identities aligned (mdv, Microsoft.Metadata.Visualizer, symbol packages, manifest)
• MergedManifest.xml confirmed PublishingVersion="4"
• Tracker: Tracking: onboard .NET Eng - Latest subscription targets to Arcade V4 publishing arcade#16697

[mmitche]

@tmat Can you take a look here and approve if it looks good?