Remove the notnull constraint from IDictionary<TKey, TValue> keys

[sharwell]

This will work around dotnet/corefx#42386 for the 3.x releases until the constraint is removed for .NET 5.

[jnm2]

The constraint should also be removed from IReadOnlyDictionary and several extension methods. This is the diff to follow: https://github.com/dotnet/runtime/commit/48a78bfa13e9c710851690621fc2c0fe1637802c

The notnull constraint was removed from the TKey generic parameter of:

• IDictionary<TKey, TValue>
• IReadOnlyDictionary<TKey, TValue>
• CollectionExtensions.GetValueOrDefault<TKey, TValue>
• CollectionExtensions.Remove<TKey, TValue>
• CollectionExtensions.TryAdd<TKey, TValue>

[jnm2]

@sharwell This can be closed now that 5.0.0 annotations are out. We might also want to update the code snippets in the readme to use 5.0.0.

[jnm2]

@sharwell Readme update is ready: #88