Update bitbucket upgrade docs (4.x to 5.x)

[Marcin Kubica]

There's a major bug in documentation, where it says 

Both ssl.key-store-password and ssl.key-password require explicit configuration. Even if you did not configure both values in the server.xml file, values for both entries need to be explicitly set in bitbucket.properties to secure Bitbucket with SSL.

https://confluence.atlassian.com/bitbucketserver/migrate-server-xml-customizations-to-bitbucket-properties-897811761.html

As the matter of fact you should not set server.ssl.key-password if you didn't use them in server.xml as it will fail the server to start with  java.security.UnrecoverableKeyException: Cannot recover key

[Ben Stuart]

Hey Marcin, I'm not sure what the scinario was that you ran into, but this line should be correct.  We added this line because when migrating server.xml settings, if server.xml only had keystorePass, springboot may require both key-store-password and key-password to be specified, if both values are other than the default changeit.