ci: avoid setup-go cache deprecation warning

[vuon9]

Summary

• remove setup-go built-in caching so CI no longer invokes the deprecated Node 20 actions/cache/restore@v4 helper
• keep the explicit Bun dependency cache on actions/cache@v5.0.5

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml"); puts "yaml ok"'\n- rg -n "cache: true|cache-dependency-path|FORCE_JAVASCRIPT_ACTIONS_TO_NODE24|actions/cache" .github/workflows/ci.yml shows only explicit actions/cache@v5.0.5 usages

[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%