Update system requirements.

[conradwt]

This PR supports the following features:

• update to Elixir 1.20.3
• update to Erlang 29.0.5

[github-actions]

Docker image tag(s) pushed:

ghcr.io/conradwt/zero-to-graphql-using-elixir:gha-30976111113
ghcr.io/conradwt/zero-to-graphql-using-elixir:pr-323 

Labels added to images:

org.opencontainers.image.created=2026-08-05T04:49:47.369Z
org.opencontainers.image.description=The purpose of this example is to provide details as to how one would go about using GraphQL with the Elixir Language.
org.opencontainers.image.licenses=MIT
org.opencontainers.image.revision=107a53f375b89ab49e65623d5aa0c11c27463b11
org.opencontainers.image.source=https://github.com/conradwt/zero-to-graphql-using-elixir
org.opencontainers.image.title=zero-to-graphql-using-elixir
org.opencontainers.image.url=https://github.com/conradwt/zero-to-graphql-using-elixir
org.opencontainers.image.version=gha-30976111113