Support only Elasticsearch 7+

[turbaszek]

Description
Support only Elasticsearch 7+.

Use case
We have many places where we check ES version and decide what to use depending if it's ES 7 or 6 and failing silently when it's something else.

This also reduces the test matrix.

Related Issues
#84

[turbaszek]

@michalslowikowski00 @Humbedooh what do you think?

[Humbedooh]

I'm doing the same for another project I work on. 7+ generally works as kibble installations are most often standalone and thus not tied to an old legacy database or the likes.

So, I'm +1 to supporting 7+ only.

[turbaszek]

@Humbedooh thanks for insight!