java/openjdk8-20: Bootstrap from prebuilt packages

Use prebuilt known good packages (derived from the official pkgs)
instead of depending on the previous version of the OpenJDK.

This patch fixes bootstrapping for openjdk8-20 except 18 which is
expired, and 19 which has been dealt with before.

PR: 291872
Reviewed by: fuz (mentor)
Approved by: fuz (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54683