[lint] remove the --preview flag when running black

Summary:
I thought using the --preview flag would make the future transition from black 23.X to 24.X smoother, but unfortunately it is not as stable as I imagined. There is no guarantee that the result of running the linter with this flag will not change on minor upgrade, as we just saw with black 23.7.0 reverting a rule for string formatting (https://github.com/psf/black/pull/3640).

The result in this diff was obtained in two steps:

• adopt the latest expected future style for multiline concatatenated strings, by first running arc lint --everything after upgrading black to 23.7.0
• remove the --preview flag from .arclint, and rerun arc lint --everything

The second step does not revert the first one because manually splitting long strings over multiple lines is already compatible with black 23.X (it just won't do it automatically for you).

Test Plan:
check we get the same result for all 23.X versions:

pip index versions black
for version in 23.1.0 23.3.0 23.7.0
do
    pip install black==${version}
    arc lint --everything
done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14294