Update Accessing services sections on Elixir, Lisp & Ruby pages

[AnouckColson]

Where on docs.platform.sh should be changed?

https://docs.upsun.com/languages/elixir.html#accessing-services-manually
https://docs.upsun.com/languages/lisp.html#accessing-services
https://docs.upsun.com/languages/ruby.html#connecting-to-services

What exactly should be updated?

Now that service environment variables have been documented via #3809, we need to update these sections that still mention the PLATFORM_RELATIONSHIPS variable.

This requires experimentation with the 3 languages to figure out how the same result can be achieved using service environment variables. Consider using codetabs to present both ways (using service environment variables vs. using PLATFORM_RELATIONSHIPS)? See with the team who wants to dig into what.

Additional context

No response