[Fleet] Adjust minimum % on auto-upgrade modal

[Supplementing]

Summary

Closes #228728

• Sets the minimum percentage of agents to upgrade to be 1% when using the arrows. In the case of a manual entry, the validation errors will now show an appropriate message about using a non-zero percentage.

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

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[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[Supplementing]

@elasticmachine merge upstream

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: a2fbcde

Failed CI Steps

• FTR Configs #61

Test Failures

• [job] [logs] FTR Configs #61 / serverless security UI Serverless Security Cases Cases List bulk actions status change the status of cases to in-progress correctly

Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
fleet	178.4KB	178.5KB	+73.0B

[jen-huang]

Btw this bugfix can be backported to whatever version the bug was found :)