Logs a warning banner at startup because the 25.2 branch is no longer
maintained.

The banner is shown in both production and development mode, as on 24.8
and older branches (#23343).

The text is new compared to the 25.0/25.1 banner (#24917):

> This is an unmaintained version of Vaadin that no longer receives
security fixes.
>
> Upgrade to a maintained version, or learn about extended maintenance
for this version on https://vaadin.com/maintenance

The new text does not suggest that the version already has known
security issues on the first day it becomes unmaintained, and the second
sentence says what the linked page is for.

Co-authored-by: Mikhail Shabarov <61410877+mshabarov@users.noreply.github.com>