Store Phoenix version and Elixir requirement in text file.

[eksperimental]

Simplify release process by storing Phoenix version and Elixir requirements in files

This PRs simplifies the release process by storing as single source of truth for:

• The Phoenix and Phoenix installer version in VERSION
• The Elixir requirement for Phoenix in ELIXIR_REQUIREMENT
• The Elixir requirement for Phoenix installer in installer/ELIXIR_REQUIREMENT

It removes the need to update the templates in the Phoenix installer. Only .json and .md are required to be updated manually.