Run both stable and preview ecosystem checks (#8422)
Closes https://github.com/astral-sh/ruff/issues/8076
Follow-up to #8358
Doubles the amount of ecosystem checks we do, adding separate groups for
the stable sections.
We're likely to run into GitHub comment length restrictions if there are
significant deviations. However, it should not be common for changes in
stable and preview to occur at the same time, nor should it be common
for linter and formatter changes to occur at the same time.
