community/openjdk8: upgrade to 3.18.0 (8u282)

This MR upgrades OpenJDK8 to the latest release. As far as I can see, no critical CVE issues where fixed.

https://mail.openjdk.java.net/pipermail/distro-pkg-dev/2021-February/042462.html

https://mail.openjdk.java.net/pipermail/jdk8u-dev/2021-January/013337.html