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:
#589
#581
#594
#609
#612
#613
#614
#615
#677
#679
#684
#685
#697
#715
#717
#720
#743
#728

[mykaul]

Let's drop 1.8 in next minor release.

Minor release is not the greatest timing for this. But perhaps given how ancient 1.8 is, it's reasonable. Would be good to provide a heads up for such changes.

There are some dependency updated that are blocked by it: #594 #589 #581

[Lorak-mmk]

Minor release is not the greatest timing for this.

There isn't really any other time to do this. If we only did that in major release (a major release would be a Java Driver 5), we would still be supporting Java 6 and older.