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

Part of issue: ash-project/ash#2745

Summary

Bumps Elixir to 1.20 and updates mix.lock to match the current Ash ecosystem.

Fixes warnings surfaced by the upgrade:

• Remove unused requires in lib and test support files
• Reorder InjectMetadata.build_metadata/2 clauses so false/nil match before the is_atom/1 catch-all
• Drop unreachable on_lookup_fields/3 clauses (Ash no longer returns those nil-action shapes)
• Namespace generated subscription config modules per resource to stop "redefining module" warnings in the test suite
• Add missing primary key/actions to test resources required by newer Ash validation

[zachdaniel]

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