Change `documentationUrl` link text in the upgrade assistant

[jportner]

When adding a deprecation with a documentationUrl field, the link text is rendered as "Learn more about this deprecation".

The doc link isn't always about the deprecation though, we should change this to be a bit more generic: "Learn more" should suffice

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)

[sabarasaba]

Fixed with #115117