fix: geojson invalid query on unavailable PostGIS

Closes https://github.com/PostgREST/postgrest/issues/4245.

It adds a query on schema cache construction, but doesn't add a new
attribute to the SchemaCache type. We only need the information to be
passed to the `initialMediaHandlers` function to build the builtin media
handlers.

test/io/__snapshots__/test_cli/test_schema_cache_snapshot[dbMediaHandlers].yaml

@@ -3,11 +3,6 @@
   - - tag: BuiltinOvAggCsv
     - tag: MTTextCSV
 
-- - - tag: RelAnyElement
-    - tag: MTGeoJSON
-  - - tag: BuiltinOvAggGeoJson
-    - tag: MTGeoJSON
-
 - - - tag: RelAnyElement
     - tag: MTApplicationJSON
   - - tag: BuiltinOvAggJson