feat: template minimum version numbers for dependencies

In #98 (closed) some of the challenges with having renovate keep dependencies fresh in this repo were discussed. In particular the contents of the template/ directory are, well, templated and so there's no guarantee that renovatebot will be able to parse them. Even if we made them parseable, it's hard to enforce continued parse-ability in CI.

Instead, lean in to the templating approach by adding files which renovatebot can parse specify the minimum version requirements and have copier read from that file to populate the requirements within the template.

Fortunately the copier installed alongside renovatebot has support for copier-templates-extensions installed which, in turn, allows adding custom extensions to the template context via Python scripts inside the template repo.

Combine both things together: we now have template*requirements.txt files which renovatebot can parse and update which are used to replace constructions such as {{ deps.somepackagename.version }} in the pyproject.toml and .gitlab/webapp.gitlab-ci.yml files. When renovatebot updates the template*requirements.txt files, the templates will automatically make use of the new minimum versions.

Note that these are always minimum versions. We don't want to stop renovate from being able to additionally update dependencies in templated repos even if a new version of the template with bumped dependencies hasn't been released.

Add some configuration to renovate.json to reduce the number of MRs likely to be raised by renovate to a manageable rate.

[1] https://github.com/renovatebot/renovate/discussions/34368