Update tests to V8 VSPG

[ckunki]

Since 2023-06-02 for version 8.18.1 of Exasol database a Docker image is available on Dockerhub.

The current ticket therefore requests to update the integration tests of VSPG to use version 8.18.1 as latest default version.

Please note sibling-tickets for all JDBC-based virtual schemas.

[pj-spoelders]

Found a possible bug in the sql compiler for CURRENT_SCHEMA pushdown:
https://exasol.atlassian.net/browse/SPOT-19716