[log4j] Ramp down for `2.26.0`

I want to release `2.26.0` in a week or two, plus 1 week for the release
process.

See the `2.26.0` milestone on GitHub:
https://github.com/apache/logging-log4j2/milestone/22

Since `2.25.4` PRs (IMO, unfortunately) targeted the branch `2.25.x`
instead of first targeting `2.x` and then getting cherry-picked from `2.x`
to `2.25.x`, those changes don't appear on the `2.26.0` milestone. Hence,
for those, you need to include the `2.25.4` milestone changes
<https://github.com/apache/logging-log4j2/milestone/25> too. I will land a
PR copying `src/changelog/2.25.4` contents to `src/changelog/.2.x.x`.

Please comment if you have any remarks.