Refactor/upgrade deprecated code usage for Mac OSX 10.15 Crypthography

[buyaa-n]

With new version of MAC OS Catalina many crypthography related methods deprecated. For now related warnings are suppresed with dotnet/corefx#39938 which need further investigion and upgrade/renewal

[bartonjs]

We have to build for the oldest platform we support, and Apple is quick to put deprecated markers on things.

#60138, amongst others, talks about upgrading our baseline, and that will probably result in switching off some old things and onto new ones, reducing suppressed warnings.