[ci] Fix remaining permanent CI warnings.

[jpobst]

Fixes various CI warnings.

Specifying an exact version is not recommended on Microsoft-Hosted agents. Patch versions of Python can 
be replaced by new ones on Hosted agents without notice, which will cause builds to fail unexpectedly. It is 
recommended to specify only major or major and minor version (Example: `3` or `3.9`)

Fix: only specify major.minor as requested.

build-tools\scripts\RunTests.targets(3,3): Warning MSB4011: "C:\a\_work\1\s\Directory.Build.props" 
cannot be imported again. It was already imported at "C:\a\_work\1\s\build-tools\scripts\PrepareWindows.targets 
(3,3)". This is most likely a build authoring error. This subsequent import will be ignored. 
build-tools\scripts\RunTests.targets(7,3): Warning MSB4011: "C:\a\_work\1\s\Configuration.props" 
cannot be imported again. It was already imported at "C:\a\_work\1\s\build-tools\scripts\PrepareWindows.targets 
(11,3)". This is most likely a build authoring error. This subsequent import will be ignored.
tests\api-compatibility\api-compatibility.targets(3,3): Warning MSB4011: "C:\a\_work\1\s\Configuration.props" 
cannot be imported again. It was already imported at "C:\a\_work\1\s\build-tools\scripts\PrepareWindows.targets 
(11,3)". This is most likely a build authoring error. This subsequent import will be ignored.

Fix: add a flag denoting that we have already imported a project and a Condition to prevent duplicate imports.

build-tools\xaprepare\xaprepare\Application\Context.cs(608,42): Warning CS8604: Possible null reference 
argument for parameter 'path1' in 'string Path.Combine(string path1, string path2)'.

Fix: add null check.

EXEC(0,0): Warning : dotnet archive URL https://dotnetcli.azureedge.net/dotnet/Sdk/9.0.100-
alpha.1.23610.1/dotnet-sdk-9.0.100-alpha.1.23610.1-win-x64.zip not found

Fix: checking multiple URLs is part of the expected flow, downgrade message to "info".

EXEC(0,0): Warning : Duplicate Third Party Notice 'IronyProject/Irony' 
(old class: Xamarin.Android.Prepare.XamarinAndroidBuildTasks_IronyProject_Irony_TPN; 
new class: Xamarin.Android.Prepare.XamarinAndroidToolsAidl_IronyProject_Irony_TPN)

Fix: multiple projects may use the same dependencies, downgrade to "info".

Remaining CI warnings are temporary because our MAUI tests are failing:

🎉

[pjcollins]

@grendello does this represent some issue that we should try to fix rather than suppressing the warning?

[grendello]

The intention was that if we see this warning, we go in and remove the duplicate, and the assumption was that it would be a very rare occurrence. So, yep, if we see it, we should remove the license duplicates (if possible) :)

[jpobst]

The warning is caused by the TPN Irony, which is used by both Xamarin.Android.Build.Tasks and Xamarin.Android.Tools.Aidl. I considered removing one of the instances but I was afraid that might cause us to miss the TPN in the future.

For example, we remove the Xamarin.Android.Tools.Aidl version today. Then in the future we remove the need for Xamarin.Android.Build.Tasks to use Irony, so we remove that version. We would not remember that we need to add the ...Aidl version back.

It felt safer to keep both and reduce the warning than risk accidentally removing them both in the future.

But I'm open to other suggestions.

[pjcollins]

LGTM aside from the one question above.