[AppKit] Fix all availability attributes in this framework.

[rolfbjarne]

• Add missing availability attributes for .NET.
• Remove !.NET availability attributes / code.
• Remove redundant .NET availability attributes.
• Don't use deprecated APIs when there's an equivalent non-deprecated version
  available.

This is a step towards fixing #21185.

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✅ [PR Build #a6b3da8] Build passed (Build packages) ✅

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✅ [PR Build #a6b3da8] Build passed (Detect API changes) ✅

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✅ [PR Build #a6b3da8] Build passed (Build macOS tests) ✅

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💻 [PR Build #a6b3da8] Tests on macOS M1 - Mac Monterey (12) passed 💻

✅ All tests on macOS M1 - Mac Monterey (12) passed.

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💻 [PR Build #a6b3da8] Tests on macOS X64 - Mac Sonoma (14) passed 💻

✅ All tests on macOS X64 - Mac Sonoma (14) passed.

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💻 [PR Build #a6b3da8] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

✅ All tests on macOS arm64 - Mac Sequoia (15) passed.

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💻 [PR Build #a6b3da8] Tests on macOS M1 - Mac Ventura (13) passed 💻

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[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

.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 )

❗ API diff vs stable (Breaking changes)

.NET ( ❗ Breaking changes ❗ )

• iOS: vsdrops gist ( ❗ Breaking changes ❗ )
• tvOS: vsdrops gist ( ❗ Breaking changes ❗ )
• MacCatalyst: vsdrops gist ( ❗ Breaking changes ❗ )
• macOS: vsdrops gist ( ❗ Breaking changes ❗ )

ℹ️ Generator diff

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

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💻 [CI Build #a6b3da8] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

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[vs-mobiletools-engineering-service2]

🚀 [CI Build #a6b3da8] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 112 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 9 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 8 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

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