Update upgrade restrictions for agents in documentation

[rseldner]

Fixed outdated restrictions verbiage on upgrading agents with respect to Fleet Server versions and minor version limitations.

I used wording that would match the equivalent 9.x doc:
https://www.elastic.co/docs/reference/fleet/upgrade-elastic-agent#upgrade-agent-restrictions

That said, I think the existing 9.x wording is a bit convoluted. It leads with the restriction, then walks it back with a "so while you can... but you cannot..." to note the permissive case of maintenance upgrades... which I think is more useful to state up front. I'd suggest something more straightforward for a future change:

{agent} can be upgraded to a newer maintenance release within its current minor version, but not to a newer minor version than {fleet-server}. For example, an 8.18.0 {fleet-server} supports upgrading {agents} up to any 8.18.x version, but not to 8.19.0. Before you upgrade {agents} to a newer minor version, you should first upgrade any agents that are acting as {fleet-server} (any agents that have a {fleet-server} policy associated with them).

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