[dotnet] Remove support for MtouchArch and XamMacArch.

[rolfbjarne]

When migrating Xamarin projects to .NET projects, somewhat frequently people
will leave the MtouchArch/XamMacArch properties in their project files with
old values. This won't work, since we use RuntimeIdentifier(s) now to control
the target architecture, so remove support for MtouchArch/XamMacArch, and show
an error if we detect that they're set.

This will hopefully prevent some really confusing problems, especially in the IDEs.

Example: #19258

[vs-mobiletools-engineering-service2]

❌ [PR Build] Tests on macOS M1 - Mac Big Sur (11.5) failed ❌

Failed tests are:

• introspection

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Ventura (13.0) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13.0) passed.

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

Pipeline on Agent
Hash: a3295de998a5af426925223e521ca4646039965d [PR build]

[vs-mobiletools-engineering-service2]

📚 [PR Build] Artifacts 📚

Packages generated

View packages

• xamarin.ios-17.1.0.50.pkg
• xamarin.mac-9.1.0.50.pkg

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS (no change detected)
• tvOS (no change detected)
• watchOS (no change detected)
• macOS (no change detected)

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

Pipeline on Agent
Hash: a3295de998a5af426925223e521ca4646039965d [PR build]

[vs-mobiletools-engineering-service2]

🔥 [CI Build] Test results 🔥

Test results

❌ Tests failed on VSTS: simulator tests

0 tests crashed, 1 tests failed, 234 tests passed.

Failures

❌ monotouch tests

1 tests failed, 40 tests passed.

• monotouch-test/watchOS 32-bits - simulator/Debug: TimedOut

Html Report (VSDrops) Download

Successes

✅ bcl: All 69 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests: All 1 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 7 tests passed. Html Report (VSDrops) Download
✅ framework: All 8 tests passed. Html Report (VSDrops) Download
✅ generator: All 2 tests passed. Html Report (VSDrops) Download
✅ interdependent_binding_projects: All 7 tests passed. Html Report (VSDrops) Download
✅ install_source: All 1 tests passed. Html Report (VSDrops) Download
✅ introspection: All 8 tests passed. Html Report (VSDrops) Download
✅ linker: All 65 tests passed. Html Report (VSDrops) Download
✅ mac_binding_project: All 1 tests passed. Html Report (VSDrops) Download
✅ mmp: All 2 tests passed. Html Report (VSDrops) Download
✅ mononative: All 6 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ mtouch: All 1 tests passed. Html Report (VSDrops) Download
✅ xammac: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 8 tests passed. Html Report (VSDrops) Download
✅ xtro: All 2 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: a3295de998a5af426925223e521ca4646039965d [PR build]

[rolfbjarne]

Test failures are unrelated (#19320, https://github.com/xamarin/maccore/issues/2450).