eth: close miner on exit (instead of just stopping) (#21992)

This ensures that all miner goroutines have exited before stopping the blockchain. 

Co-authored-by: Felix Lange <fjl@twurst.com>

eth/backend.go

@@ -554,7 +554,7 @@ func (s *Ethereum) Stop() error {
 	s.bloomIndexer.Close()
 	close(s.closeBloomHandler)
 	s.txPool.Stop()
-	s.miner.Stop()
+	s.miner.Close()
 	s.blockchain.Stop()
 	s.engine.Close()
 	rawdb.PopUncleanShutdownMarker(s.chainDb)