CC-2081: Update Elixir to 1.19

[andy1li]

CC-2081: Update Elixir to 1.19

---

Note

Bumps Elixir to 1.19 across configs and adds a Dockerfile for 1.19 with dependency setup and caching.

• Build/Runtime:
  • Update buildpack to elixir-1.19 in compiled_starters/elixir/codecrafters.yml and solutions/elixir/01-at4/code/codecrafters.yml.
  • Add dockerfiles/elixir-1.19.Dockerfile (based on elixir:1.19.4-alpine) that installs Hex/Rebar, compiles deps, runs .codecrafters/compile.sh, and caches _build/deps.

^{Written by Cursor Bugbot for commit c0a770a. This will update automatically on new commits. Configure here.}

[linear]

CC-2081 Upgrade Elixir to 1.19