Move the Go toolchain pins to 1.26, in CI and in every image

Two Dependabot PRs are stuck behind the same number.

#35 raises the go directive to 1.26.0 in six modules, because
golang.org/x/crypto v0.56.0 requires it -- x/crypto tracks the two most
recent Go releases and 0.56 dropped 1.25. A module that says 1.26 cannot
be built by the 1.25 this repository pins in two places, so that PR
fails every Go job.

#29 raises actions/setup-go to v7, which sets GOTOOLCHAIN=local. With
that set, `go install golang.org/x/vuln/cmd/govulncheck@latest` cannot
quietly fetch a newer toolchain, and stops with

  golang.org/x/[email protected] requires go >= 1.26.0 (running go 1.25.14)

Under setup-go v5 the same install succeeded by downloading 1.26 behind
our backs, which is its own reason to be on 1.26 deliberately instead.

So: security-scan's go-version and all eight Dockerfiles move together,
1.25 -> 1.26. Nothing else needs to. A newer toolchain builds an older
directive happily, so this stands on its own before #35 lands, and the
go.mod files stay where they are here.

Checked by building rather than by reading: the api and ingest images
both build on golang:1.26-alpine, and api, ingest and enterprise still
`go build ./...` clean against their existing 1.25 directives.

enterprise/cmd/enterprise-api/Dockerfile

@@ -2,7 +2,7 @@
 # context must be the repo root (needs both enterprise/ and proto/, like
 # api/Dockerfile does for api/ + proto/), not enterprise/ alone.
 #   docker build -f enterprise/cmd/enterprise-api/Dockerfile -t cairnobs-enterprise-api .
-FROM golang:1.25-alpine AS builder
+FROM golang:1.26-alpine AS builder
 WORKDIR /src
 COPY . .
 WORKDIR /src/enterprise