Remove usage of "backend" term

[mosabua]

Currently the codebase and docs often refers to "backends". This is confusing since it is NOT the backend database of Trino Gateway. Instead this refers to the Trino clusters that sits behind Trino Gateway and to which queries are routed. The codebase and docs also uses Trino cluster, and clusters already.

In a dev sync a while ago the team decided to get rid of the usage of "backend" and just consistently use "Trino cluster" or "cluster". "Trino" can be omitted when it is clear form the context. Other forms in code like camel case or whatever are also fine .. just the "backend" should go.