fix(api): add CVSS_V4 to API Swagger documentation (google#2486)

The addition of `CVSS_V4` in google#2312 did not add it to the Swagger
documentation.

Open to suggestions on how to guard against such divergence in the
future.

Fixes: google#2485

docs/osv_service_v1.swagger.json

@@ -336,6 +336,7 @@
       "type": "string",
       "enum": [
         "UNSPECIFIED",
+        "CVSS_V4",
         "CVSS_V3",
         "CVSS_V2"
       ],
@@ -627,4 +628,4 @@
     }
   },
   "host": "api.osv.dev"
-}
+}