Remove discrepancy between prefix_key and prefix_search

In api_data.py, we define the configuration prefix_search to enable early exit on a prefix search in search_near. However, we have been using prefix_key to enable that option, see cur_std.c and one of our search_near test.

We need to change the name in api_data.py as we may have users who already use the configuration name prefix_key.

It would be worth investigating how we ended up with this discrepancy.

Definition of done:

The discrepancy is removed and one consistent terminology is used.

Make sure it does not impact MongoDB tests by checking the source code and triggering a patch build.