ci: add Elixir 1.19 and OTP 28 coverage

[savonarola]

Summary

• add Elixir 1.19 / OTP 28 combinations to the CI matrix
• bump the existing Elixir 1.18 entry to the latest 1.18 patch release
• run the test job in MIX_ENV=test so test-only tooling is used consistently
• keep Credo out of test builds while making Dialyxir available in test for CI dialyzer runs
• refresh checkout/cache actions and align the code quality job with the latest stable toolchain

Validation

• parsed .github/workflows/elixir.yml successfully
• ran MIX_ENV=test mix format --check-formatted && MIX_ENV=test mix test && MIX_ENV=test mix dialyzer in elixir:1.19.5-otp-28 successfully

[coveralls]

coverage: 92.408%. remained the same — ci/elixir-1-19-otp-28-matrix into main