ci: pin json below 3 for Rails 6.0 through 8.0 appraisals - #136

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5, ruby/setup-ruby@a4effe49ee8ee5b8b5091268c473a4628afb5651. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/