Remove <LangVersion>preview from empty web template

[halter73]

This shouldn't be necessary to get C# 10 features any more.

[DamianEdwards]

Change to enable this was just merged to Roslyn so once it flows to the SDK we should be OK to make this change in main.

[DamianEdwards]

Latest SDK (6.0.100-preview.7.21356.3 at time of writing) has this change now. Not sure if this has flowed to ASP.NET repo though.

[DamianEdwards]

OK just verified this works now with 6.0.100-preview.7.21329.3 which is what the repo is (at least in my branch) so I'll go ahead and update this as part of #34016