Update formatting for Elixir 1.15

[axelson]

Description

Update formatting for Elixir 1.15 (mainly the charlist format update, which is backwards compatible with earlier elixir versions)

Also enforce formatting in CI

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature
  but make things better)

Checklist

• Check other PRs and make sure that the changes are not done yet.