PLAT-19 - add support for OTP 24 / elixir 1.12

[gharnly]

The crypto functions in OTP have been revised with deprecations introduced in OTP 23 and the old versions have been completely removed in OTP 24. See: https://www.erlang.org/doc/general_info/deprecations.html#otp-23

As such this PR:

• Adds support for both the old and the new API which will allow this package to be used in systems running OTP 24 and higher
• Relaxed the elixir requirement so that systems using this package can upgrade to elixir 1.12 or higher

QA
I tested using asdf to have multiple local versions of elixir running. Add a .tools-versions file to the root of the project with these contents:

erlang 24.3.4.4
elixir 1.12.3-otp-24

Then run mix test. On master this will fail but on this branch it will pass.

To confirm it still works on elixir 1.11 / OTP 23 update your .tools-versions to:

erlang 23.3.4.5
elixir 1.11.4-otp-23

and run mix test

[jangromko]

Checked locally, with both versions of Elixir and OTP project has been compiled correctly, tests passed and docs have been generated without any issues — everything looks good. 🦭