Fix CI matrix support range and release workflow validation

[pcharbon70]

Summary

• align CI matrix with declared support range in project/mix.exs (elixir: ~> 1.18)
  • remove failing unsupported lane: Elixir 1.17.3 / OTP 26.2
  • keep/add supported lanes: 1.18.4/27.3 and 1.19.0/28.0
• fix release workflow validation by avoiding direct secrets.* usage in if expressions
  • move secrets to job env
  • gate steps with if: env.* != ''

Why

• current main CI run fails in unsupported Elixir lane
• release workflow check run is failing at startup with no jobs, indicating workflow file validation issue