fixes OpenAPI spec security definitions

SpiceDB does not use ApiKeyAuth authentication, but Bearer authentication, where
the type of bearer token is an API Key.

However, the OpenAPI v2 Spec, which is the one supported by grpc-gateway,
does not support bearer authentication:
https://swagger.io/docs/specification/v2_0/authentication/authentication/

Still, the grpc-gateway maintainers indicated in
grpc-ecosystem/grpc-gateway#1089
that bearer is actually supported in grpc-gateway generator.

This was reported in authzed/authzed-go#255,
indicating that folks generating code out of the OpenAPI definition
will have errors because the generated error did not properly provide
the preshared key with the expected `Authorization: Bearer <psk>`
format.

I'm not 100% sure if this is a legit intermediate state
between v2 and v3 we can leverage, but the current generated
code is clearly broken anyway.

See https://swagger.io/docs/specification/v3_0/authentication/api-keys/
See https://swagger.io/docs/specification/v3_0/authentication/bearer-authentication/

authzed/api/v1/openapi.proto

@@ -32,13 +32,18 @@ option (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_swagger) = {
 	produces: "application/json";
 	security_definitions: {
 		security: {
-			key: "ApiKeyAuth";
+			key: "bearer";
 			value: {
 				type: TYPE_API_KEY;
 				in: IN_HEADER;
 				name: "Authorization";
+				description: "SpiceDB preshared-key, prefixed by Bearer: Bearer <preshared-key>"
 			}
 		}
 	}
+	security: {
+		security_requirement: {
+			key: "bearer"
+		}
+	}
 };
-