Use Astral's stable Python 3.15 Docker image

[adamtheturtle]

Use ghcr.io/astral-sh/uv:0.13.1-python3.15-trixie-slim for all three mock services. The base image supplies final Python 3.15 and CA certificates, so remove the manual Python installation, installation-directory settings, and Debian certificate installation. Add a release note.

Fixes #3731.

Validation:

• Image builds and all six existing Docker deployment tests pass on both linux/arm64 and linux/amd64; amd64 runs under emulation with Buildx-built images.
• Both images report Python 3.15.0, run as UID 10001, and pass a standard-library HTTPS request with certificate verification enabled.
• File-scoped pre-commit and pre-push hooks, including Hadolint and manifest checks, pass.