Solved How to Install a Package without its Runtime Dependencies.

You can't instruct pkg to use a other dependency than written in the package. And: The port itself has no dependency to openjdk8; Instead the port asks at build time for the actual default version - and that is 8. So only the package sets that dependency.
By setting DEFAULT_VERSIONS+=openjdk=11 in /etc/make.conf and installing the port instead of using the package should (untested!) do what you want.