Fix ci

[jhchen]

Note

Low Risk
Low risk: only CI configuration and README badge links change, with no production code impact. Potential risk is CI failures due to updated Elixir/OTP matrix or workflow filename change.

Overview
Updates the GitHub Actions CI workflow by replacing the old ci.yml (master branch, legacy Elixir/OTP versions, older action versions) with a new ci.yaml targeting main, testing against modern Elixir/OTP combos, and bumping checkout/cache/setup actions.

Updates README build badge and workflow links to point at the new ci.yaml workflow path.

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