[Fleet] Enhancement - Allow typing version in agent upgrade modal

[criamico]

Currently the only way to choose the version for upgrading agents is to select it from a select in the agent upgrade modal:

There have been cases where the endpoint fetching the versions has failed, the desired version is not available in the dropdown list.

• Replace the select with a combobox which would also allow to type in the desired version:

• Add a message specifying the minimum version available for upgrade/downgrade (7.17.0)
• Add a link to the available versions: https://www.elastic.co/downloads/past-releases#elastic-agent

The combobox needs some validation on the version but the endpoint will check the passed version too.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[jlind23]

@hop-dev isn't it you fixed recently?

[juliaElastic]

Yes, it was fixed here: #165978