CI: run tests across Go 1.21–1.27

[carlostasada]

Replace the compile-only matrix and pinned 1.24 test job with go test on every supported toolchain. Drop 1.20 (below the module floor) and leave lint on 1.24 until golangci-lint is migrated to v2.

[greptile-apps]

Confidence Score: 5/5

The workflow change appears safe to merge, with no concrete correctness, compatibility, or security regression identified.

Summary

• Runs the complete test suite on every supported toolchain.
• Removes Go 1.20 because it is below the module’s declared minimum.
• Keeps linting on Go 1.24 for compatibility with golangci-lint v1.64.8.

_{Reviews (1) · Last reviewed commit: "ci: run tests across Go 1.21–1.27"}