Bump golang in /examples/http-gateway

Bumps golang from 1.24-bookworm to 1.26-bookworm.

---
updated-dependencies:
- dependency-name: golang
  dependency-version: 1.26-bookworm
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

examples/http-gateway/Dockerfile

@@ -2,7 +2,7 @@
 # Build context is the repository root:
 #   docker build -f examples/http-gateway/Dockerfile -t piguard-gateway .
 
-FROM golang:1.24-bookworm@sha256:1a6d4452c65dea36aac2e2d606b01b4a029ec90cc1ae53890540ce6173ea77ac AS build
+FROM golang:1.26-bookworm@sha256:5d2b868674b57c9e48cdd39e891acce4196b6926ca6d11e9c270a8f85106203d AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download