Adopt lock-file SciTools/workflow CI

[HGWright]

See

• https://github.com/SciTools/workflows
• https://github.com/SciTools/iris/blob/main/.github/workflows/refresh-lockfiles.yml
• https://scitools-iris.readthedocs.io/en/latest/developers_guide/github_app.html

Our current thinking is that:

• SciTools has organisation secrets configured
• SciTools has organisation GH App configured

Actions:

• Create a requirements/ci directory and move all *.yml files into it
• Create the .github/workflows/refresh-lockfiles.yml (see SciTools/iris or SciTools/iris-grib)

Blockers:

• The SciTools reusable GHA creates a PR with the generated *.locks and places them in the requirements/ci/nox.lock directory - this is now not applicable to repos that don't use nox, plus we are migrating from nox to tox.