Remove android-net8 workload

[Copilot]

We temporarily added support for net8.0-android projects in .NET 10:

• aa0485f

We no longer need this, as .NET 8 MAUI is out of support: https://aka.ms/maui-support-policy

This PR removes:

• The android-net8 workload definition from WorkloadManifest.in.json
• The Microsoft.Android.Sdk.net8 pack definition
• .NET 8.0 framework imports and references from WorkloadManifest.in.targets
• android-net8 from workload installation arguments in Directory.Build.targets
• net8.0-android test cases that were building .NET 8 Android projects

The changes are minimal and surgical, removing only the specific .NET 8 Android workload support while preserving all other functionality.

Fixes #10227.

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[jonathanpeppers]

We should also remove all instances of $(AndroidNet8PreviousVersion).

There are also some tests that do things like:

		static readonly object[] DotNetPackTargetFrameworks = new object[] {
			new object[] {
				"net8.0",
				"android",
				34,
			},

We should remove the test cases like this for net8.0.

[Copilot]

We should also remove all instances of $(AndroidNet8PreviousVersion).

There are also some tests that do things like:
...

Fixed in commit a854f42. Removed all instances of $(AndroidNet8PreviousVersion) from eng/Versions.props and build-tools/create-packs/Microsoft.NET.Sdk.Android.proj, and removed the net8.0 test cases from both DotNetPackTargetFrameworks and DotNetTargetFrameworks arrays in XASdkTests.cs.

[jonathanpeppers]

This looks good, but I'm going to wait until we branch for the next .NET 10 preview to merge this.

@copilot make no further changes, but do a quick verification that this is the inverse of: aa0485f

Check if we missed anything.

[Copilot]

This looks good, but I'm going to wait until we branch for the next .NET 10 preview to merge this.

@copilot make no further changes, but do a quick verification that this is the inverse of: https:/...

I've verified that the changes are the inverse of commit aa0485f. The cleanup successfully removes:

✅ All AndroidNet8PreviousVersion references from eng/Versions.props and build tools
✅ The android-net8 workload definition and pack from WorkloadManifest.in.json
✅ .NET 8.0 framework imports and references from WorkloadManifest.in.targets
✅ android-net8 from workload installation arguments
✅ net8.0 test cases from both DotNetPackTargetFrameworks and DotNetTargetFrameworks arrays
✅ net8.0-android from device integration test parameters

The changes precisely undo the temporary .NET 8 support that was added. All net8.0 references have been removed while preserving net9.0 and net10.0 support.