Mailing list threading improvements

TL;DR: We=E2=80=99re updating how auto-generated email from Github will be
threaded on your mailing lists. If you want to keep the old defaults,
details are below.

We=E2=80=99re pleased to let you know that we=E2=80=99re tweaking the way t=
hat auto-
generated email from Github will appear on your mailing lists. This
will lead to more human-readable subject lines, and the ability of most
modern mail clients to correctly thread discussions originating on
Github.

Background: Many project mailing lists receive email auto-generated by
Github. The way that the subject lines are crafted leads to messages
from the same topic not being threaded together by most mail clients.
We=E2=80=99re fixing that.

The way that these messages are threaded is defined by a file -
.asf.yml - in your git repositories. We=E2=80=99re changing the way that it
will work by default if you don=E2=80=99t choose settings. If you=E2=80=99r=
e happy for
us to make this change, don=E2=80=99t do anything - the change will happen =
on
October the 1st 2023.

Details of the current default, as well as the proposed changes, are on
the following page, along with instructions on how to keep your current
settings, if you prefer:

https://community.apache.org/contributors/mailing-lists.html#configuring-th=
e-subject-lines-of-the-emails-being-sent

Please copy [email protected]<mailto:[email protected]>
on any feedback.

Chris, on behalf of the Comdev PMC