Build elixir-devel against runtime25

From: Pat Maddox <pat_at_patmaddox.com>
Date: Tue, 13 Dec 2022 02:35:31 UTC
I want to build elixir-devel against erlang-runtime25.

The Makefile hardcodes ERLANG_VER at 24: 
https://cgit.freebsd.org/ports/tree/lang/elixir-devel/Makefile#n37

Should this instead be

     ERLANG_VER?= 24

to still default to 24, but to respect a different version set in 
make.conf ?

I changed that one line in my local ports tree, and it built against 
runtime25, everything works as expected.

Pat