ci: remove Node 20 apt cache action

[vuon9]

Summary

• replace the APT cache action with direct apt installs in CI jobs
• keep Bun caching on actions/cache@v5.0.5
• removes the remaining actions/cache/restore@v4 Node 20 warning source

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml"); puts "yaml ok"'\n- rg -n "cache-apt-pkgs-action|actions/cache/restore|actions/cache|Install Linux dependencies|Cache APT" .github/workflows/ci.yml

[github-actions]

Package	Line Rate	Health
devtoolbox/internal/barcode	93%	✔
devtoolbox/internal/codeformatter	80%	➖
devtoolbox/internal/converter	65%	➖
devtoolbox/internal/datagenerator	81%	✔
devtoolbox/internal/datetimeconverter	75%	➖
devtoolbox/internal/jwt	47%	❌
devtoolbox/internal/numberconverter	98%	✔
devtoolbox/internal/settings	0%	❌
devtoolbox/internal/themes	0%	❌
Summary	70% (3726 / 5294)	➖

Minimum allowed line rate is 60%