Fix JDK references in contributing docs (#34669)
Fix JDK references in contributing docs (#34669)

After we bumped the JDK compiler minimum requirements to JDK 11, we
missed updating the contributing docs to point to JDK 11 instead of JDK
10. This commit fixes that.

Additionally, when we stopped requiring JDK 7 to run the reindex from
old tests, we did not update the contributing docs to reflect this. This
commit also fixes that.
HonzaKral/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,HonzaKral/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch