4.x: drop support for java 1.8

[dkropachev]

Let's drop 1.8 in next minor release.

There are some dependency updated that are blocked by it: https://github.com/scylladb/java-driver/pull/589 https://github.com/scylladb/java-driver/pull/581 https://github.com/scylladb/java-driver/pull/594 https://github.com/scylladb/java-driver/pull/609 https://github.com/scylladb/java-driver/pull/612 https://github.com/scylladb/java-driver/pull/613 https://github.com/scylladb/java-driver/pull/614 https://github.com/scylladb/java-driver/pull/615 https://github.com/scylladb/java-driver/pull/677 https://github.com/scylladb/java-driver/pull/679 https://github.com/scylladb/java-driver/pull/684 https://github.com/scylladb/java-driver/pull/685 https://github.com/scylladb/java-driver/pull/697 https://github.com/scylladb/java-driver/pull/715 https://github.com/scylladb/java-driver/pull/717 https://github.com/scylladb/java-driver/pull/720 https://github.com/scylladb/java-driver/pull/743 https://github.com/scylladb/java-driver/pull/728