Introduce IReproducibleRng to make RNG Create and CreateSeed methods accessible by static interface methods

Add static Create and CreateSeed methods to each of the reproducible RNGs

Introduce ISelfSeedingRng to make RNG Create methods accessible by static interface methods

.NET 7's static abstract lets us finally put static methods in interfaces. This lets us pass just the type to a method needing an RNG instead of a factory instance.

For example:
void Set<TRng>() where TRng : IRng, ISelfSeedingRng<TRng> => _rng = TRng.Create();
instead of
void Set<TRng>(IRngFactory<TRng> factory) where TRng : IRng => _rng = factory.Create();

Suppress some errors

Bump lang version to 11 and analysis level to 7

Add ChaCha256-specific tests

Use hardware-agnostic Vector128 and Vector256 instrinsics methods on .NET 7 instead of requiring AVX2 or SSE2

These have equivalent performance to their AVX2 and SSE2 counterparts, with the added benefit of potentially providing acceleration on non-x86 platforms such as ARM (this needs to be verified).

Add .NET 7 as a target

Clean up and simplify ChaChaIntrinsics

Simplify PropertyGroup conditions

Drop support for .NET 5

Use latest version of BenchmarkDotNet

Suppress Rider/ReSharper's inconsistent naming warning

Implement ISeekableBlockRngCore on ChaCha intrinsics implementations

Fix minor warnings

Bump analysis level to 6

Update C# version to 10 and use file-scoped namsepaces

Fix missed conditions for .NET 4.8

Add benchmarking for .NET 6

Make build script Linux-friendly by using cross-platform New-Item commandlet and
bump tested .NET Framework version to 4.8