Change RSA.LegalKeySizes for Apple platforms to return 8192

[vcsjones]

This changes RSA.LegalKeySizes to return a maximum of 8192 on Apple platforms, as opposed to 16384. At least on macOS Sequoia and Sonoma, the limit is 8192-bit.

While we are here, memoize the KeySizes instance. They are immutable. The returned array is not - so we only memoize the interior KeySizes instance, not the returned array.

Closes #110588.

[teo-tsirpanis]

Should we add a test that keys with the max size can be created?

[vcsjones]

Should we add a test that keys with the max size can be created?

We do here

runtime/src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/RSA/KeyGeneration.cs

Lines 23 to 27 in cc66c77

	[ConditionalFact(typeof(TestEnvironment), nameof(TestEnvironment.IsStressModeEnabled))]
	public static void GenerateMaxKey()
	{
	GenerateKey(rsa => GetMax(rsa.LegalKeySizes));
	}

But it's (effectively) a manual-only because it takes about ~30 seconds for macOS to generate an RSA 8K key. It can take several minutes for platforms that support 16K keys. Even for Outerloop that's too long, in my opinion.

This manual-only test fails without the fix, which is what prompted me to make the change.

[teo-tsirpanis]

Makes sense, thanks.