[Fleet] GA release for Agent binary upgrade feature

[mostlyjason]

The agent binary upgrade feature is currently experimental and we'd like to bring this feature up to a GA release. We need to make some breaking changes so that its consistent with our licensing strategy. We also need to improve to operational stability of the feature. This issue is a placeholder, pending full definition.

Problems:

• Update behavior based on our licensing strategy
• Support rolling upgrades to prevent overloading the network
• Currently, it switches from updating to healthy status if the upgrade fails. The only record of failure is in the logs.
• Decide on licensing for [Fleet] Ability to configure agent.download.sourceURI in Kibana #100413

Related

• [Agent] Improve error log messages when agent binary cannot be downloaded elastic-agent#104
• [Fleet] Give the ability to set up elastic agent download timeout #121069
• [Fleet] Automatic Agent Upgrade option in Agent Policy #120735
• [Fleet]: Upgrade toast shows wrong agents count on upgrading agents through bulk options under Agents tab. #127422
• [Fleet]: Upgraded Agent toast instead of Upgrading Agent when upgrade is triggered for lower version Agent. #116742

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[jen-huang]

Closing in favor of an internal project issue; new public-facing issues to come.