Avoid use of deprecated SecTrustGetCertificateAtIndex

[csstaub]

Avoid use of deprecated SecTrustGetCertificateAtIndex. Replaces #420.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.42%. Comparing base (3a8fd6e) to head (c6b685e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
+ Coverage   78.40%   78.42%   +0.02%     
==========================================
  Files          31       31              
  Lines        2135     2137       +2     
==========================================
+ Hits         1674     1676       +2     
  Misses        381      381              
  Partials       80       80              

Flag	Coverage Δ
darwin	77.47% <100.00%> (+0.02%)	⬆️
linux	83.31% <ø> (-0.19%)	⬇️

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[csstaub]

Tested, this worked fine for me.

Easy way to create test keys for this:

• Open the Keychain Access app
• Go to Keychain Access (menu) => Certificate Assistant => Create a Certificate
• Create various certificates, certificate authorities, etc. whatever you want to test