Update build and test environment to Elixir 1.20. Update other dependencies too.

[cbecker]

Checklist

• I confirm that all text in the PR description, commit messages, code comments, and documentation is human-authored.

Description

• Updates Dockerfile to build with Elixir 1.20
• Fixes warnings obtained with Elixir 1.20
• Forces meck (only for tests) to 1.0, as the 0.9.2 required by mock won't compile with OTP 29
• Updates other libraries that had update candidates

@achempion let me know if you'd like to not upgrade certain libs. Tests pass.

[achempion]

looking great, thanks for the PR

It seems that we need to update the credo config as well so it doesn't fail (it should be possible to disable the moduledoc rule).

[cbecker]

looking great, thanks for the PR

It seems that we need to update the credo config as well so it doesn't fail (it should be possible to disable the moduledoc rule).

Updating credo to the latest version fixed it. Also fixed remaining credo issues.