ci: drop Go 1.24 from matrix (go.mod requires Go 1.25)

[stakahashy]

Summary

• drop '1.24' from the ci job's Go version matrix in .github/workflows/test.yml
• bump AGENTS.md Go style guideline from Go 1.24+ to Go 1.25+ to match

Prepare v3.27.0 (420dd780) bumped go.mod from go 1.24.0 to go 1.25.0 and removed the toolchain directive, making Go 1.25 the new minimum. The CI matrix in test.yml and the Go Code Style line in AGENTS.md were not updated in the same change, so every PR opened against v3 since then has been failing on ci (1.24, ubuntu-latest) with:

# goa.design/goa/v3/jsonrpc
go: no such tool "covdata"
# goa.design/goa/v3/middleware/xray/xraytest
go: no such tool "covdata"
# goa.design/goa/v3/security
go: no such tool "covdata"

Go 1.24's toolchain cannot build a module that declares go 1.25, so the go test ./... --coverprofile=cover.out step fails for packages that contain no test files (where the build falls back to the coverage-instrumentation path that needs the covdata tool).

Examples: #3929, #3930 — both fail on 1.24 and pass on 1.25 despite having no coverage-related changes, confirming the failure is toolchain-gated rather than test-logic-specific.

The Upload test coverage for deep source step already gates on matrix.go == '1.25', so dropping 1.24 has no side effect there.

Test plan

• confirm go.mod declares go 1.25.0
• confirm the artifact upload step is gated on Go 1.25 and is unaffected
• CI on this PR runs only on Go 1.25 and goes green (verified after push)

[raphael]

Good catch, thank you. I've updated the branch protection required checks as well.