[release/8.0.1xx] [tests] Fix certificate tests after Apple started b…

…locking the Thawte certificate. (#19665)

The “Thawte SGC CA” certificate we're using for testing is now blocked
by
Apple, evaluating trust results in this error:

> “Thawte SGC CA” certificate is blocked

So update the mail_google_com certificates to what mail.google.com
serves now, and also update the corresponding parent certificates (which
have changed).

Then adjust the tests accordingly.

Finally delete all unused certificates from the code.

Fixes these test failures:

    MonoTouchFixtures.Security.TrustTest
        [FAIL] Trust_FullChain :   GetTrustResult-2
            Expected: RecoverableTrustFailure
            But was:  FatalTrustFailure
at MonoTouchFixtures.Security.TrustTest.Trust_FullChain(SecTrust trust,
SecPolicy policy, X509CertificateCollection certs) in
/Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line
389
at MonoTouchFixtures.Security.TrustTest.Trust_FullChain() in
/Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line
321
        [FAIL] Trust2_FullChain :   GetTrustResult-2
            Expected: RecoverableTrustFailure
            But was:  FatalTrustFailure
at MonoTouchFixtures.Security.TrustTest.Trust_FullChain(SecTrust trust,
SecPolicy policy, X509CertificateCollection certs) in
/Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line
389
at MonoTouchFixtures.Security.TrustTest.Trust2_FullChain() in
/Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line
449

Backport of #19650.