lang/elixir-devel: update to 1.16.0

• https://elixir-lang.org/blog/2023/12/22/elixir-v1-16-0-released
• require OTP26 for build and runtime

In the past, the Elixir compiler performance was largely
independent of the version of Erlang/OTP used to build
Elixir itself. This is no longer the case, so the OTP
version has been bumped, and users will need to use
the latest OTP version for this -devel port.