Update system requirements.

[conradwt]

This PR supports the following features:

• update to Erlang 28.1.1
• update to Elixir 1.19.1
• update to PostgreSQL 18.0

[github-actions]

Docker image tag(s) pushed:

ghcr.io/conradwt/zero-to-graphql-using-elixir:gha-18694314748
ghcr.io/conradwt/zero-to-graphql-using-elixir:pr-280 

Labels added to images:

org.opencontainers.image.created=2025-10-21T18:48:20.926Z
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=64d831620a738ae436bf19223fb8788bd9fbd453
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-18694314748