chore: refresh test matrix to Elixir 1.20 / OTP 29

[yordis]

• Track the latest stable Elixir/OTP combo as the lint-gating entry so the project stays aligned with current releases.

[cursor]

PR Summary

Low Risk
Only GitHub Actions workflow versions change; no application or library code is affected.

Overview
Updates the GitHub Actions Test workflow in .github/workflows/test.yml so CI tracks newer Elixir/OTP releases.

The main Test matrix drops Elixir 1.16.3 / OTP 26.2.5, keeps 1.17.1 / 27.0 and 1.19.0 / 28.0, and adds 1.20.0 / 29.0.1 as the sole matrix entry with lint: true (format check). Test-gun1 and dialyzer jobs move from 1.16.3 / 26.2.5 to 1.17.1 / 27.0 for beam setup.

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[Copilot]

Pull request overview

Updates the CI test workflow matrix so the lint/format-gating job runs on the latest Elixir/OTP combination, keeping the project aligned with current BEAM releases.

Changes:

• Adds an Elixir 1.20.0 / OTP 29.0.1 matrix entry.
• Moves the lint: true gating flag from Elixir 1.19.0 / OTP 28.0 to the new latest entry.

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