cmd/utils, node: switch to Pebble as the default db if none exists (#27136)

* cmd/utils, node: switch to Pebble as the default db if none exists

* node: fall back to LevelDB on platforms not supporting Pebble

* core/rawdb, node: default to Pebble at the node level

* cmd/geth: fix some tests explicitly using leveldb

* ethdb/pebble: allow double closes, makes tests simpler

cmd/utils/flags.go

@@ -101,8 +101,8 @@ var (
 	}
 	DBEngineFlag = &cli.StringFlag{
 		Name:     "db.engine",
-		Usage:    "Backing database implementation to use ('leveldb' or 'pebble')",
-		Value:    "leveldb",
+		Usage:    "Backing database implementation to use ('pebble' or 'leveldb')",
+		Value:    node.DefaultConfig.DBEngine,
 		Category: flags.EthCategory,
 	}
 	AncientFlag = &flags.DirectoryFlag{