Update keystore type JKS to PKCS12 in documentation where appropriate

While JKS can still be used and is not deprecated, starting from JDK9 the default keystore format is PKCS12. This issue is for updating the occurences of JKS to PKCS12 where appropriate in the documentation. These can be names of keystore files that were generated with keytool commands, or default types used in subsystem commands etc.