Update MacOS keychain support to use non-deprecated API

[faec]

As mentioned in #2639, most build and test actions on Mac produce a lot of noisy warning text related to the deprecation (in MacOS 10.10) of the old SecKeychain functions in Security.framework. We can suppress the build warnings for short-term convenience, but the oldest MacOS we support is 10.13, so we no longer need to support this deprecated API -- we should migrate our keychain handling to the current supported alternative.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[jlind23]

@pchila is this something you touched touch while working on #3866?

[pchila]

@jlind23 I didn't touch the internal implementation of the Mac OS keychain vault, I only separated the 2 implementations of vaults we have so that we can use both on darwin (see #4362).

If we need to change the implementation to move away from deprecated APIs there should be no conflicts (also, it still needs to be done 😅 )