### Description
<!-- Describe your changes. -->
Update some testing dependencies.
Fix various warnings. Mainly around documentation (existing) and unit
test usage (mainly resulting from xunit update).
Invalid angle brackets for generics in documentation were changed to use
curly braces based on
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/
> To refer to generic identifiers in code reference (cref) elements, you
can use either the escape characters (for example, cref="List<T>")
or braces (cref="List{T}"). As a special case, the compiler parses the
braces as angle brackets to make the documentation comment less
cumbersome to the author when referring to generic identifiers.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
### Description
<!-- Describe your changes. -->
Xamarin is EOL so remove support.
The MAUI targets are EOL and need updating.
https://dotnet.microsoft.com/en-us/platform/support/policy/maui
Other cleanups:
- netcoreapp3.1 is EOL
- the net6 macos target was added in the mistaken belief that was for
MAUI mac support, but that is actually via the mac-catalyst target which
we recently added support for.
- some CIs that were using the old build setup of splitting pre-net6
targets. The ORT C# bindings csproj was updated last year and the
`PreNet6` and `SelectedTargets` properties no longer exist as they were
replaced by the simpler `IncludeMobileTargets` property.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Remove EOL components.
#21058
### Description
1. use the pool with VS2022
2. upgrade System.Memory to 4.5.5
### Motivation and Context
Solve the build error while using VS2022:
`[Failure] Msbuild failed when processing the file
'D:\a\_work\1\s\csharp\src\Microsoft.ML.OnnxRuntime\Microsoft.ML.OnnxRuntime.csproj'
with message: Method not found: 'System.ReadOnlySpan`1<Char>
Microsoft.IO.Path.GetFileName(System.ReadOnlySpan`1<Char>)'`
Ref:
https://stackoverflow.com/questions/73399777/azure-build-failing-due-to-method-not-found-system-readonlyspan1char-micros
### Description
<!-- Describe your changes. -->
Various fixes to the CSharp setup
- fix warnings
- fix invalid tests
- update test sdk nuget package
- enables testing on linux
- fixes issue with some unit tests not running in CI
- run unit tests in linux pipeline using dotnet
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Unit tests weren't breaking in CIs for both Windows and Linux builds and
should have been.
### Description
<!-- Describe your changes. -->
Add infrastructure so it's easy for a user to add the ORT extensions
nuget package and register the custom ops for C# apps.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Need to be able to use extensions on mobile platforms with Xamarin/MAUI
Add Xamarin support to the ORT nuget packages.
- Update C# code to support Xamarin builds for iOS and Android
- refactor some things to split out common code
- include iOS and Android ORT native shared library in native nuget package
