For gpb 5: Drop support for Erlang versions before 19

[tomas-abrahamsson]

This is an advance notice that I plan to drop support for Erlang versions before 1819 and at the same time bump gpb's major version to 5.

Edit: Adding that I'm also thinking of changing an option-default from {verify, optionally} to {verify, always}. I'm also thinking of changing the default json_format to maps, since there is a json module about to make its way into Erlang 27. Currently gpb already defaults to maps when generating code that uses maps, but it defaults to eep18 when generating code that uses records. It is this last use-case with records that would change default.