Is there any open and updated POLYGON data of world cities for postgis (or another standard format) I need to convert lot/lon to city by querying in our database , I don't want to use external services
A great source for administrative borders is GADM. There you can find administrative borders from the whole world in many administrative levels and data formats, such as shapefiles and geopackage.
If you wondering how to import the GADM data into PostGIS, take a look at this other answer.