[net9.0] Migrate Messaging Build agent to .NET 8

[rolfbjarne]

This is part of the effort to migrate the Pair to Mac agents .NET.

As the Xamarin.iOS.Tasks.Windows project targets netstandard2.0, I'm
removing the Build agent reference, and modifying the Makefile to take
it from it's output directory. Note: the agent zip file is generated in
the intermediate output directory.

[rolfbjarne]

These changes had a bumpy start:

1. First added to main (in Migrate Messaging Build agent to .NET 8 #20705).
2. main got merged into net9.0, so they ended up in net9.0 as well.
3. Then they got reverted in main, because they're only supposed to be in .NET 9 (and we might release new .NET 8 updates from main. This happened in Revert "Migrate Messaging Build agent to .NET 8 (#20705)" #20790.
4. main got merged into net9.0, bringing the revert with the merge.

So here we re-apply the changes into net9.0.

[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: a18e4680d51b2b43ea30f541e4f8121c9f3793ff [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.

Pipeline on Agent
Hash: a18e4680d51b2b43ea30f541e4f8121c9f3793ff [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.

Pipeline on Agent
Hash: a18e4680d51b2b43ea30f541e4f8121c9f3793ff [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

NET (empty diffs)

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

✅ API diff vs stable

.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)

ℹ️ Generator diff

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

Pipeline on Agent
Hash: a18e4680d51b2b43ea30f541e4f8121c9f3793ff [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 99 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. [attempt 2] Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. [attempt 2] 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 1 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. [attempt 2] 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. [attempt 2] Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 8 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 9 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: a18e4680d51b2b43ea30f541e4f8121c9f3793ff [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

Pipeline on Agent
Hash: a18e4680d51b2b43ea30f541e4f8121c9f3793ff [PR build]