Warning:The JKS keystore uses a proprietary format. It is recommended to migrate to PKCS12 which is an industry standard format using "keytool -importkeystore -srckeystore mywebsite.jks -destkeystore mywebsite.jks -deststoretype pkcs12".
This is just a warning. It can be simply ignored without any issues. Either you can use ".keystore" in the output filename instead of ".jks" to get rid of this warning.
Use .keystore extention