print/pdf-renderer: allow building with jdk21

Error was:
[00:01:29] -do-compile:
[00:01:29] [javac] Compiling 116 source files to /wrkdirs/usr/ports/print/pdf-renderer/work/pdf-renderer-0.9.1_1,1/build/classes
[00:01:30] [javac] warning: [options] bootstrap class path not set in conjunction with -source 5
[00:01:30] [javac] error: Source option 5 is no longer supported. Use 8 or later.
[00:01:30] [javac] error: Target option 5 is no longer supported. Use 8 or later.
[00:01:30]
[00:01:30] BUILD FAILED

PR: 291576