Proj4031: Avoid global diagnostic severity configuration

Using dotnet_diagnostic.severity to force a universal baseline level introduces significant friction to a codebase. Every time any of the enabled analyzers adds a new rule that was not intended for general usage, and therefore disabled by default is now included.

Furthermore, you lose all granularity that is built-in by the builders of the analyzers. Not all rules are created equal.

Non-compliant

is_global = true
        
dotnet_diagnostic.severity = suggestion

Compliant

is_global = true

dotnet_diagnostic.SA1649.severity     = none
dotnet_diagnostic.SA1512.severity     = suggestion
dotnet_diagnostic.SYSLIB1045.severity = warning