chore: bump Go to 1.26.4

[dc-tec]

Summary

Bumps the repository Go toolchain pin from 1.26.3 to 1.26.4.

This updates the canonical Go version in go.mod, refreshes the pinned golang:1.26.4 builder image digest across the project Dockerfiles, and updates the contributor setup prerequisite documentation.

Related Issues

None.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (code improvement/cleanup)
• CI, release, or build tooling
• Other maintenance

Risk and Compatibility

Low. This is a Go patch-version bump and builder image refresh. No API, CRD, Helm, RBAC, security policy, upgrade flow, or runtime behavior change is intended.

Verification

• go mod tidy
• GOFLAGS='-mod=mod' go mod vendor && git diff --exit-code -- vendor
• make doctor
• GOFLAGS='-mod=vendor' go test $(GOFLAGS='-mod=vendor' go list ./... | grep -v '/test/e2e')
• Pre-push hook: make lint-ci

make ci-core was not run locally.

Reviewer Notes

I did not add .go-version because the repository already uses go.mod as the Go version source for GitHub Actions and make doctor, keeping a single source of truth.

Historical perf baseline JSON files and versioned website docs still mention Go 1.26.3; those were left unchanged because they describe captured/historical artifacts rather than active toolchain pins.

Checklist

• My code follows the project style guide.
• I have performed a self-review of my own code.
• I have added or updated tests, or explained why tests are not needed.
• I have updated documentation, or explained why docs are not needed.
• I have updated generated artifacts, or confirmed none are affected.
• I have checked that this change does not log or expose secrets, tokens, credentials, keys, or raw Secret data.
• I have run the relevant local checks, or documented why they were not run.
• Any dependent changes have been merged, published, or clearly called out.

[dc-tec]

Supersedes #496