Address build warnings from .NET 8 analyzers

[jimmylewis]

• Mostly CA1307 to specify a StringComparison on string operations. Most of these were fixed with conditional compilation, but I suppressed a couple where it wouldn't matter (non-cased characters) and the code would have been ugly to use conditional compilation.
• Some HashCode implementations were changed from XOR of two values to using HashCode.Combine instead for .NET 8.