mix releases should be required (or suggested) for Elixir 1.15

[beerlington]

I recently tried upgrading our application to Elixir 1.15 and ran into an issue where the app wouldn't boot up due to a recompile happening after a restart. More details about that can be found here https://github.com/elixir-lang/elixir/issues/12771

I was able to get around the issue by switching to mix releases (i.e. adding release=true to elixir_buildpack.config).

I'm not sure if the README should be updated or what, but I wanted to point it out in case anyone else runs into this issue, since it's not the default configuration.