Adjust `CommonTemplatesTests` to net8 changes in templates

[JanKrivanek]

#27486
#28707

CommonTemplatesTests will need to accomodate changes - specifically:

• net8.0 TFM needs to be added to test matrix
• add tests for new item templates synonyms: .gitignore, global.json, nuget.config, .editorconfig (ideally while sharing snapshots with existing shortnames to verify identity of output)

Related work: adding C# 12 to test matrix: dotnet/templating#6566

[JanKrivanek]

Adressed by 3ccb7ba

[JanKrivanek]

.net6.0 X C# >= 11.0 and default cases are removed from the new test matrix temporarily (dotnet/templating#5668)

[vlada-shubina]

Already addressed.