chore: bump Go 1.26.1 → 1.26.6

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.

Start by locating every go.mod and the GO_IMG digest entries in build/images.mk, .pipelines/build/scripts/install-go.sh, and bpf-prog/ipv6-hp-bpf/linux.Dockerfile; retrieve the Microsoft Go 1.26.6 digest as instructed. Regenerate pipeline Dockerfiles with make dockerfiles, run go mod tidy from the root, and verify completion with go build ./...; the auto/go-patch-1.26.6 branch provides a reference diff.

Written by the indexing model from the issue text.