Add a preset to manage updates to PYTHON_VERSION is CI files

Description

The boilerplate application defines a python language version in the CI files. By default, this is not managed by renovate.

Add a custom manager maintain the python version in the CI files.

Further details

The boilerplate even mentions:

# Make sure this matches the version in pyproject.toml and the Dockerfile.

Which feels like a job for renovate!

Acceptance criteria

• Add a preset to add the ability for renovate-bot to manage the python version defined to CI files.
• Comments raised in renovate-config!24 (closed) have been reviewed and approved against the changes.