update runtime to elixir-1.17.1 and erlang-27.0

[ruslandoga]

Changes

This PR updates Elixir to 1.17 and Erlang to 27 mostly to bring in Duration. I picked the base image with the smallest alpine version difference (3.18.6 vs 3.18.4) but we can update it as well (to 3.20).

mjml_eex (akoutmos/mjml_eex#19) and credo had to be updated to make them work again.

Warnings from the new Elixir type checker for phoenix_live_view failed CI tests so I backported phoenixframework/phoenix_live_view@489e8de in 0d07f4d. This workaround can be removed once Plausible starts using phoenix_live_view 1.0.0 (currently in rc).

There are warnings from ex_money and phoenix_view too but they don't seem to fail CI so I left them as is.
Later ex_money can be updated to 5.17.0 and phoenix_view -- to 2.0.4.

Tests

• Tests have been adapted

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI