Replace usage of obsolete certificate APIs

[bart-vmware]

In .NET 9, some .NET APIs for loading certificates have been obsoleted. Can we replace them with something else?

Search the codebase for SYSLIB0057 and usage of X509Certificate2.CreateFromPemFile(string, string) and new X509Certificate2(string) for existing suppressions.

[TimHess]

#1571 included some potential changes around X509RevocationMode that would be worth revisiting during this effort. IIRC there was a reason that's currently defaulting to offline, but it is worth investigating a bit further to see if there's anything we could do different to be either more secure by default or easily enable tighter security