[UIKit] Improve code for UISegmentedControl. Fixes #21289.

[rolfbjarne]

1. Enable nullability.
2. Move Get|SetTitleTextAttributes to generated code.
3. Remove dead code (pre-.NET code paths).

Point 2. fixes #21289, so add tests for this scenario.

Fixes #21289.

[vs-mobiletools-engineering-service2]

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[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS X64 - Mac Sonoma (14) passed 💻

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

Pipeline on Agent
Hash: 982b6c43caaf0efd4dd283e550ab925b475a2fa8 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Big Sur (11) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11) passed.

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Hash: 982b6c43caaf0efd4dd283e550ab925b475a2fa8 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Monterey (12) passed 💻

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

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Hash: 982b6c43caaf0efd4dd283e550ab925b475a2fa8 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Ventura (13) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13) passed.

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Hash: 982b6c43caaf0efd4dd283e550ab925b475a2fa8 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

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Hash: 982b6c43caaf0efd4dd283e550ab925b475a2fa8 [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 97 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. [attempt 2] 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. [attempt 3] Html Report (VSDrops) Download
✅ generator: All 1 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 40 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 7 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 7 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 7 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

Pipeline on Agent
Hash: 982b6c43caaf0efd4dd283e550ab925b475a2fa8 [PR build]

[rolfbjarne]

/sudo backport net9.0

[vs-mobiletools-engineering-service2]

Backport Job to branch net9.0 Created! The magic is happening here

[vs-mobiletools-engineering-service2]

Hooray! Backport succeeded! Please see https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=10312177 for more details.