Draft: Elixir 1.15 compatibility

• Adding a changelog: In the changelog.d directory, create a file named <code>.<type>.
• Logger warn→warning
  • Config migration?
• Add missing leading / on requests (might be linked to dependency update though)
• Logger backends: https://github.com/elixir-lang/logger_backends/issues/4
• Hard to reproduce errors in various functions like Pleroma.Web.MediaProxy.whitelisted?/1 with IO.chardata_to_string/1 getting a nil
• mix format changes, CI update?