Infra: move iOS/Mac Catalyst sample CI jobs from macos-15 to macos-26 #622

Summary

Move the macOS jobs in the sample build workflows from macos-15 to macos-26, and re-evaluate the iOS workload / Xcode pins that exist only because of macos-15.

Background

• While validating Infra: upgrade actions/setup-dotnet from v5 to v6 #617 (PR ci: upgrade actions/setup-dotnet from v5 to v6 #621), three jobs in the Build Samples workflow never started: todoapp-maui / ios, todoapp-avalonia / ios and todoapp-uno / ios-maccatalyst. GitHub reported The job was not acquired by Runner of type hosted even after multiple attempts, with the annotation Due to capacity constraints, jobs targeting macOS arm64 runners may experience longer queue times. A rerun of the failed jobs hit the same error. See run 42231478149.
• The runner-images macOS directory lists macos-26 as the newest generally available image (macOS 26.6.2) and a separate xcode-27-arm64 image (macOS 27, Xcode 27 public preview).
• Newer images are likely, but not guaranteed, to have more spare runner capacity than macos-15. GitHub does not publish pool sizes, so this needs to be confirmed empirically.

Why the current setup is constrained by macos-15

The three iOS jobs (build-samples-todoapp-maui.yml, build-samples-todoapp-avalonia.yml, build-samples-todoapp-uno.yml) all use runs-on: macos-15 and carry workarounds from #529 and #559:

• WorkloadSetVersion: '10.0.107', pinned because newer sets (10.0.108+) need the iOS SDK 26.5 pack, which requires Xcode 26.5.
• RequiredXcodeVersion: '26.3' plus a sudo xcode-select -s step, because the macos-15 default Xcode is 16.4 and its newest Xcode is 26.3.

The macos-26 image (20261026.0455.2) installs Xcode 26.0.1, 26.1.1, 26.2, 26.3, 26.4.1, 26.5 and 26.6 (default). Xcode 26.5 is the version that #529 and #559 found missing, so the workload-set pin may no longer be needed.

Proposed change

1. Change runs-on: macos-15 to runs-on: macos-26 in the three workflows above.
2. Update the explanatory comments, which currently refer to macos-latest migrating and to macos-15.
3. Decide on the workload / Xcode strategy:
   • Minimal: keep WorkloadSetVersion: '10.0.107' and RequiredXcodeVersion: '26.3' (Xcode 26.3 is still installed on macos-26). This only changes the runner.
   • Follow-up: remove the 10.0.107 pin and select Xcode 26.5 or newer, so CI tracks the current .NET iOS SDK pack. Do this only if the iOS builds still pass.
4. Dispatch build-samples.yml on the fork and confirm that the ios and ios-maccatalyst jobs start (acquire a runner) and pass.

Out of scope

• Xcode 27 / the xcode-27-arm64 image. It is a public preview and is not yet a standard runs-on label. Revisit once it is generally available.
• Linux and Windows runner changes (Ubuntu was handled in ci: pin Ubuntu runner image instead of ubuntu-latest (migrating to Ubuntu 26.04) #612).

Acceptance criteria

• No macos-15 references remain in .github/workflows/.
• Build Samples passes on the fork, including the iOS and Mac Catalyst jobs.
• Comments in the workflows accurately describe the runner and Xcode choices.

Risks

• macos-26 may have its own capacity constraints; this is not known until tried.
• The default Xcode, the OpenSSL default (3 to 4, see runner-images#14856) and tool versions differ from macos-15; the iOS builds should be re-verified.