### 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
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
We want to implement SoftmaxCrossentropy and NegativeLossLikelihoodLoss forward training ops for opset-12 but that requires ONNX submodule to point to the latest commit to have the latest and greatest ONNX spec!
- Reverse integrate changes from *.in.proto files in github ONNX repo.
- Regenerate csharp/test/Microsoft.ML.OnnxRuntime.Tests/OnnxMl.cs
- Disable ONNX tests that don't have op implementation for the latest opset.
* Initial commit
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* Remove C# ifdefs
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* YAML changes for other release pipelines
* Add release notes metadata
* Props and Targets change
* Add CSHarp proj
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* Minor fix
* Minor fix
* Fix yaml
* Some missing logic for winml
* Minor update
* Fix casing for winmd file
* Fix casing
* Add targets and props for managed section into native nuget
* revert file
* a
* Initial check in
* Add win x86
* minor update to x86
* update win-ci
* update win-ci
* update win-x86ci
* add linux and mac templates
* add nuget pipelines and test templates
* remove buildConfig
* add compliance template
* fix minor typos
* update pool for macos
* update mac agent pool
* update macos pool
* update agent pools for tests
* turn off debug build for testing
* some modifications to packaging scripts
* change ordering of compliance tasks
* Add mklml pipeline
* Add packagename variable to mklml pipeline
* remove unrequired dependent jobs from mklml pipeline
* Update build command for macOS legs in mklml and cpu pipeline
* Set vcvars to true
* Add no contrib ops pipeline
* Add no-contrib-ops pipeline
* set vcvars to true for package tests
* remove repetition in nuget templates
* get buildarch correct
* get name of test template correct
* remove steps from test_all_os.yml
* add parameters to test_all_os.yml
* Need jobs, not steps
* set envars for disablecontrib ops
* add cleanup tasks and CG to package tests
* fix path to cleanup script for macos
* remove buildDirectory -- not needed
* remove fp16tiny_yolov2 model from nocontribops tests
* remove debugging info
* fix individual linux pipelines to use correct template
* remove unneeded bak_latest2
* increase timeout to 120 to allow for variance
* turn off code coverage report
