Dynamically match Elixir major/minor versions

[Nezteb]

This PR accomodates newer versions of Elixir by not hard-coding specific major/minor verions. Based on this conversation in the Discord.

[github-actions[bot]]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days

[coffee-cup]

Closing in favour of https://github.com/railwayapp/nixpacks/pull/935