Drop support for Erlang/OTP 20.3 and earlier

[aronisstav]

Is your feature request related to a problem? Please describe.
It is desirable to drop support for earlier versions of Erlang/OTP, in order to eliminate special code, test handling and reduce the time needed to run the test suites.

Describe the solution you'd like
Drop support for Erlang/OTP 20.3 and earlier.

Describe alternatives you've considered
It is not too pressing to make this decision now, but at some point we do have to stop supporting older versions in the interest of maintenance, and this is as good time as any.

Additional context
Suggested by a user in #315 .