[templates] Default $(SupportedOSPlatformVersion) to 24.

[jpobst]

Fixes: #9517
Context: dotnet/android-libraries#767
Context: dotnet/android-libraries#767 (comment)

For .NET 10, update the dotnet new android template so that $(SupportedOSPlatformVersion) is 24, bumping from the current default of 21.

The reason for this is that "desugaring" "moves" Java methods to locations that we don't expect, which can result in AbstractMethodErrors at runtime. Setting the minimum SDK version to >= 24 avoids this desugaring step, preventing the Java methods from being moved in a manner we don't expect, and thus avoiding the AbstractMethodError.

Note that 21 will still be the supported minimum for those that need it, however this will keep most users who do not need to support devices that old from having desugaring issues.

[jpobst]

This "fixes" the issue going forward. Should we also add a warning when $(SupportedOSPlatformVersion) < 24 to encourage users to update their existing projects?

Edit: I guess this is part of the #9527 proposal.