It says what to do on the Java doc (I recommend not using the Kotlin documentation for much; it’s not really maintained well):
versionCode
This field was deprecated in API level 28. Use getLongVersionCode() instead, which includes both this and the additional versionCodeMajor attribute. The version number of this package, as specified by the tag’s versionCode attribute.
This is an API 28 method, though, so consider using PackageInfoCompat. It has one static method:
getLongVersionCode(PackageInfo info)