Update to Elixir 1.20.0 and refresh Hex packages.

[conradwt]

This PR supports the following features:

• update to Elixir 1.20.0
• refresh Hex packages

[github-actions]

Docker image tag(s) pushed:

ghcr.io/conradwt/zero-to-graphql-using-elixir:gha-27084593892
ghcr.io/conradwt/zero-to-graphql-using-elixir:pr-315 

Labels added to images:

org.opencontainers.image.created=2026-06-07T06:13:01.776Z
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=ea6a8b9263f82260261b5606f85ec2e924432eb7
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-27084593892