[Foundation] Fix nullability in NSMutableDictionary<TKey, TValue>.

This is file 40 of 47 files with nullability disabled in Foundation.

* Enabled nullable reference types.
* Removed four SupportedOSPlatform attributes without version numbers.
* Replaced 'To be added.' XML comments with proper documentation.
* Added XML documentation for all public and protected members.
* Used ArgumentNullException.ThrowIfNull for parameter validation.
* Fixed XML comment formatting and added see cref attributes.
* Made ObjectForKey return nullable TValue? since keys may not exist.
* Made TryGetValue parameter nullable with NotNullWhen(true) attribute.
* Kept IDictionary interface implementations non-nullable per .NET contract.
* Used KeyNotFoundException when indexer accesses missing keys.
* Fixed IEnumerator implementations to skip null values in enumeration.
* Changed empty array initialization from 'new TValue[]{}' to '[]'.
* Remove some dead code.
* Add a few more tests.

Contributes towards #17285.

Author: web-flow