find explain in legacy query path on mongos does not retry on StaleShardVersion

(function() { "use strict";   const dbName = "test"; const collName = "foo"; const ns = dbName + "." + collName;   const st = new ShardingTest({mongos: 2, shards: 1, verbose: 2});   let staleMongos = st.s0; let freshMongos = st.s1;   jsTest.log("Make the stale mongos load a cache entry for db " + dbName + " once"); assert.writeOK(staleMongos.getDB(dbName).getCollection(collName).insert({_id: 1}));   jsTest.log("Call shardCollection on " + ns + " from the fresh mongos"); assert.commandWorked(freshMongos.adminCommand({enableSharding: dbName})); assert.commandWorked(freshMongos.adminCommand({shardCollection: ns, key: { "_id": 1 }}));   jsTest.log("Ensure the shard knows " + ns + " is sharded"); assert.commandWorked(st.shard0.adminCommand({_flushRoutingTableCacheUpdates: ns, syncFromConfig: true}));   jsTest.log("Run explain find on " + ns + " from the stale mongos"); // Expect the test to fail here. staleMongos.getDB(dbName).getCollection(collName).find({$query: {}, $explain: true}).next();   st.stop(); })();