Update `.tool-versions` to Elixir 1.20 across the Ash ecosystem

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### Is your feature request related to a problem? Please describe.

This issue tracks the work required to upgrade the Ash ecosystem to Elixir 1.20.

The goal is to update supported Elixir versions, ensure CI passes on Elixir 1.20, resolve any compiler warnings or test failures, and make any changes necessary to fully support Elixir 1.20 across the ecosystem.

This issue serves as a central tracking issue for work spanning multiple repositories.

Scope: This effort is limited to repositories that are currently published on Hex, with `ash_storage` included.

### Describe the solution you'd like

Update the .tool-versions file in all in-scope repositories to use Elixir 1.20.

This effort is limited to repositories published on Hex along with `ash_storage`.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by identifying the Hex-published Ash repositories in scope, including ash_storage, and inspect each repository’s .tool-versions file and CI configuration. Update the Elixir version references, run the existing CI checks and tests, and resolve any Elixir 1.20 compiler warnings or test failures. Done means all in-scope repositories support Elixir 1.20 with passing CI.

Written by the indexing model from the issue text.