chore: bump Go 1.26.1 → 1.26.6

[behzad-mir]

Go 1.26.6 fixes stdlib CVEs (GO-2026-6218, GO-2026-6090, GO-2026-6091, GO-2026-5972) that are failing govulncheck on all PRs.

Instructions

1. Update go 1.26.1 to go 1.26.6 in every go.mod file in the repo
2. Get the new Microsoft Go image digest by running:

   skopeo inspect docker://mcr.microsoft.com/oss/go/microsoft/golang:1.26.6-azurelinux3.0 --format "{{.Digest}}"
   

3. Update the digest in build/images.mk for the GO_IMG variable
4. Update the digest in .pipelines/build/scripts/install-go.sh
5. Update the digest in bpf-prog/ipv6-hp-bpf/linux.Dockerfile
6. Run make dockerfiles to regenerate all pipeline Dockerfiles
7. Run go mod tidy in the root directory
8. Verify the build passes with go build ./...

Reference

The branch auto/go-patch-1.26.6 has an example of these changes if you need to reference what the diff should look like.

[github-actions]

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