chore(ci): bump GitHub Actions to latest major versions

actions/checkout v4 -> v7, actions/setup-go v5 -> v7,
docker/login-action v3 -> v4, docker/setup-buildx-action v3 -> v4,
docker/setup-qemu-action v3 -> v4, docker/build-push-action v6 -> v7.
goreleaser/goreleaser-action was already on the current major.

setup-go v6 forces GOTOOLCHAIN=local, so the pinned go-version must
stay in sync with src/go.mod or builds fail instead of downloading a
toolchain; both are 1.26.0. Removed inputs in build-push-action v7 and
setup-buildx-action v4 are unused here, and checkout v7's fork-PR block
only affects pull_request_target/workflow_run, which these workflows
do not use.

Also aligns the stale Go version in AGENTS.md with go.mod.