Only create an openCypher connection for Neptune Analytics

A new notebook instance targeting Neptune Analytics will create a set of default connections in Graph Explorer for each query language. But Neptune Analytics only supports openCypher.

It should only create a single connection for openCypher. This will ensure the user is defaulting to the correct working path on first launch.

## Proposed solution

Use the existence of `GRAPH_EXP_SERVICE_TYPE` with the value `neptune-graph` as the condition to only create a single connection.

Locate the code that creates default Graph Explorer connections for new notebook instances, then inspect how GRAPH_EXP_SERVICE_TYPE is used. The change is complete when Neptune Analytics creates only an openCypher connection while other targets retain their existing default connections.

Written by the indexing model from the issue text.