Instead of referencing the shared CI repo on `main`, pin it to the
initial tagged version `v0.0.1`, so the CI behavior is stable and
does not silently change when the shared repo is updated.
First iteration of a migration toward a centralized repository
containing our shared CI logic.
Goals:
* Manage GitHub Actions version upgrades in one place.
* Make it easier to audit what actually runs in CI from a security
perspective.
* Avoid duplicating CI logic across projects and having each
repository slowly diverge over time.
* Centralize as many of our custom GitHub Actions as possible,
including some that still live in the old `numerique-gouv`
organization.
* Centralize the Renovate configuration alongside the workflows.
Inspired by the Accounts project, which recently simplified and
reorganized its CI setup.
This PR starts moving meet's CI to the shared repository so we can
validate the approach on a real project. For now, reusable
workflows are pinned to `main`; once we agree on the structure and
content of the central repository, they should be pinned to a
specific commit SHA instead.
lebaudantoine