Use Xcode 26.6 in CI

[davidortinau]

Description

Select Xcode 26.6 in both MAUI sample build workflows. The final .NET 11 Preview 7 Apple packs require Xcode 26.6.

Evidence

• Published Microsoft.iOS.Sdk.net11.0_26.5 version 26.5.11997-net11-p7 sets RecommendedXcodeVersion to 26.6.
• The dotnet/macios release/11.0.1xx-preview7 branch sets XCODE_VERSION=26.6.
• The macos-26 runner image lists Xcode 26.6 as the default at /Applications/Xcode_26.6.app.

Scope

• Change only the existing xcode-select path in build-pr.yml and build-all.yml.
• Do not change SDK selection, workload installation, build logic, or sample code.

Validation

• Parsed both workflows as YAML.
• Confirmed no workflow references /Applications/Xcode_26.3.app.
• Completed pre-push review with no findings.

[davidortinau]

@jfversluis Release-day CI unblock. Xcode 26.6 selection now passes macOS and Windows. This fixes the final shared-workflow blocker for #776 and #777; human approval is the only remaining gate.

[davidortinau]

@jfversluis Thank you for the approval. Repository policy prevents the author from merging, auto-merge is disabled, and both checks are green. Please merge this PR so #776 and #777 can rerun on Xcode 26.6.

[davidortinau]

@jfversluis All checks are now green, including the final rerun. Normal author merge remains blocked by repository policy and auto-merge is disabled. Please perform the squash merge; #776 and #777 are waiting on this exact workflow change.

[davidortinau]

@conceptdev Release-day unblock: this approved two-line Xcode selector fix is green, but repository policy prevents author merge and auto-merge is disabled. If available, please merge so the two Preview 7 samples can rerun.

[jfversluis]

@davidortinau put your copilot spam genie back in the bottle 😂