Switch image back to default Ubuntu 20.04 (#5985)
Switch image back to default Ubuntu 20.04 (#5985)

## What is the goal of this PR?

Recent version of default GraknLabs Ubuntu image already has Java 11 as default, so specialized version (`-java11`) should no longer be used.

## What are the changes implemented in this PR?

Use `graknlabs-ubuntu-20.04` as image for all jobs
lolski/grakn,lolski/grakn,graknlabs/grakn,graknlabs/grakn,lolski/grakn,lolski/grakn,graknlabs/grakn,graknlabs/grakn