Update query used to check for Cassandra connectedness

[boguth]

I discovered rather late that DESCRIBE KEYSPACES is specifically a cqlsh command and might not be supported by every Cassandra version.

Using SELECT * FROM system_schema.keyspaces queries Cassandra's system schema directly - going around any tool-specific language - and is indicated for Cassandra versions 5.0 and higher.

[boguth]

cc: @lucasmdrs

[lucasmdrs]

Thank you for checking this @boguth ! I was unaware of the incompatibility too..