java/openjdk: Fix pkgsuffix for jre builds

java/openjdk25 and openjdk26 would use the same pkgsuffix when building
the jre FLAVOR, making the package names clash.

Reported by: Antoine Brodin <antoine@freebsd.org>
Reviewed by: fuz
Approved by: fuz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56123