Bump Elixir to 1.20 and Erlang to 29.0 in CI - #30

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

using commas as separators in "mix do" is deprecated, use + between commands instead

missing specification for function is_visible_ascii/1

missing specification for function is_visible_latin1/1

missing specification for function is_iso646_basic/1

missing specification for function is_ascii/1

missing specification for function is_latin1/1

missing specification for function is_bmpchar/1

missing specification for function is_unicode/1

missing specification for function is_iso10646/1

"xref: [exclude: ...]" in your mix.exs file is deprecated, instead use: "elixirc_options: [no_warn_undefined: ...]"