Replace deprecated SecTrustGetCertificateAtIndex API in System.Security.Cryptography.Native.Apple

[akoeplinger]

When bumping to macOS12/iOS12.2 in #101342 we got a new warning about this API being deprecated.

From @filipnavara

The SecTrustGetCertificateAtIndex deprecation would need a new code path to use SecTrustCopyCertificateChain. To make that efficient we would likely need to rewrite the managed -> native interface.

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[vcsjones]

As noted in #101342 (comment), I have a head start on deprecation replacements so I can just include this one in the work.