[ci] Add variable to include or exclude current TFMs in CI builds

### Description

Currently, the Directory.Build.props file has a variable IncludePreviousTfms that controls whether previous TFMs are included in the build. To support CI scenarios where we want to selectively include or exclude the current target frameworks (TFMs), please add a new variable (e.g., IncludeCurrentTfms) that allows us to control whether the current TFMs are included in the build process.

This would enable us to easily integrate new TFMs into our CI by toggling this variable, similar to how we can control previous TFMs. The relevant code is at https://github.com/dotnet/maui/blob/net10.0/Directory.Build.props#L16.

### Public API Changes

Add a new MSBuild property:

```xml
true
```
- This property should default to true (to preserve existing behavior).
- The build logic should honor this variable similarly to how IncludePreviousTfms is used.
- Update build scripts and documentation to mention the new variable.

### Intended Use-Case

By allowing us to include or exclude current TFMs with a variable, we can control which target frameworks are built in CI for experimentation, migration, or optimization purposes. For example, when introducing a new TFM, we might want to test it in isolation or exclude it temporarily from production builds.