Migrate to C# 10

[timheuer]

Take an opportunity to migrate to new C# language capabilities like:

• file-scoped namespaces (We don't use multiple per file) #241
• new() where appropriate #242
• Evaluate where switch expressions make more readable #243

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stevenpears-bjss]

re-opening as covered by #244 - just waiting for approval