chore: update to Elixir 1.20 and clean build suite

[febarnett3]

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

Summary

Is a part of this issue: ash-project/ash#2745

• Update the project .tool-versions to Elixir 1.20
• Clean up compiler/type warnings surfaced by Elixir 1.20
• Remove unused test/support requires so the build suite runs cleanly
• Fix custom test type error handling to match its actual cast_input/2 return shape

Verified

• mix test passes
• mix check passes

[zachdaniel]

LMK when you take this out of draft and its ready 🙇 (comment, as just moving out of draft I don't think will send me a notification)

[febarnett3]

Moved out of draft. Thanks for the review! 🙂

[zachdaniel]

🚀 Thank you for your contribution! 🚀