Disable Upgrade Assistant UI by default in 8.0

[cjcenizal]

This UI isn't functional in 8.0, so we should remove it from the Kibana UI. We'll re-enable it once it can do something useful again.

[elasticmachine]

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

[cjcenizal]

Chatted with @alexfrancoeur a bit and we agreed that this direction makes more sense than leaving it enabled in the UI with a placeholder message. Reasons:

• We're not sure yet what kind of functionality we're going to provide in UA, so it's difficult to provide the right messaging in the placeholder prompt to set appropriate user expectations.
• The worst case UX of removing it is very mild. A user might wonder where Upgrade Assistant went, but they're not going to lose access to any data or have a disrupted workflow. As such this doesn't qualify as a breaking change.