databases/mysql97-client: Add maintainer note per new ports policy

The maintainer load and port lifecycle policy only excludes a port from the
outdated ratio when the reason is documented in the port itself:

https://wiki.freebsd.org/Ports/MaintainerLoadAndPortLifecyclePolicy

This is a slave port of databases/mysql97-server, but repology counts it as a
package of its own. At the time of writing it lists 26.7.0 as the newest MySQL
and reports this port as outdated, although it carries 9.7.2, the latest
release of the series it tracks. Refer to the note in the master port instead
of repeating it here.

No functional change, comment only.

Sponsored by: Netzkommune GmbH