chore(ci): reduce runner sizing to 4 core and free tier (#17785)

- In order to reduce CI costs, experiments were run on each of the
altered workflows to understand the duration (and thus impact to DX) and
changes in cost.
- The two cases of concern are what checks are run on each PR commit and
in the merge queue:
- in the merge queue- there was a trivial change in runtime. This is
because the bottleneck workflow is the mac unit test, which takes about
1 hour to run. All runner-size-reduced workflows in this PR that are in
the merge queue, still run within that time frame.
- on each PR commit- this PR does not affect this case. However, a
sister PR (#17724) does, because that PR fundamentally changes the
integration test workflow to introduce the runner there, and so in that
PR, we reduced to 4 core. That change adds 3 minutes of duration to the
integration tests, which is trivial (and integration tests only run on
some PR pushes, if that PR touches integrations).
- Thus the empirically anticipated outcome is a ~20% reduction in cost
with a none-to-trivial increase in duration.

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Co-authored-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>