ci: pin actions to commit SHAs, add Trivy, Dependabot and release-please - #1

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, hashicorp/setup-terraform@b9cd54a3c349d3f38e8881555d616ced269862dd. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/