Bump Cocoa to 7.1.3 and use PrivateSentrySDKOnly instead of the private headers

[marandaneto]

This is a private class for our SDK that exposes some stuff that is needed, eg for envelopes #391
that are more things that can be removed I guess eg the private header

[ueman]

I think @denrase already done it in 60cde6d

[denrase]

Yeah, needed to bump in order to use the new API.

[marandaneto]

The new class has more methods used by the sdk, so the bump is indeed done but not removing the old private header hacky solution

[marandaneto]

eg https://github.com/getsentry/sentry-dart/blob/main/flutter/ios/Classes/SentrySDK%2BFlutterPrivate.h could be removed in favor of PrivateSentrySDKOnly

[marandaneto]

I know the bump has been done already on #391 but that PR lands only on v6 and I'd like to bring this improvement into 5.1 too