[TheoBen880]

Moves CI and the Docker builder from Go 1.23, which is no longer supported, to Go 1.25.

• CI: go-version changes from '1.23' to '1.25'.
• Dockerfile: builder changes from golang:1.23-alpine to golang:1.25-alpine, and runtime from alpine:3.20 (end of life) to alpine:3.24.
• Build, vet and tests pass with Go 1.25.14. govulncheck now reports 0 vulnerabilities, down from 31 reachable Go standard library vulnerabilities on 1.23.
• trivy, builder image: 22 Critical / 439 High → 0 / 2.