[Fleet] Attempting to upgrade Fleet Server results in misleading warning

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Kibana version: 8.15.0

Elasticsearch version: 8.15.0

Describe the bug:

Attempting to upgrade a Fleet Server Agent from 8.14.3 -> 8.15.0 results in a misleading warning:

Please choose another version. Cannot upgrade to version 8.15.0 because it is higher than the latest fleet server version 8.14.3.

This my only Fleet Server Agent, and clicking Upgrade Agent (despite the warning) succeeds.

Also observed when attempting to upgrade the Fleet Server Agent to 8.14.x (as reported here)

Steps to reproduce:

1. Deploy 8.14.3 Stack w/ single Fleet Server Agent
2. Upgrade Elasticsearch + Kibana to 8.15.0
3. Attempt to upgrade the Fleet Server Agent to 8.15.0 to receive a misleading warning.

Screenshots (if relevant):

Any additional context: