Use JDK 10 for 6.4 BWC builds (#32866)
Use JDK 10 for 6.4 BWC builds (#32866)

The 6.4 release will officially be built with JDK 10. We should push JDK
10 down to the BWC builds for the 6.4 branch then too.
robin13/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,gfyoung/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch