[8.10](backport #3473) [Standalone Agent] Disallow upgrade if upgrade is already in progress

[mergify]

This is an automatic backport of pull request #3473 done by Mergify.
Cherry-pick of bc1982a has failed:

On branch mergify/bp/8.10/pr-3473
Your branch is up to date with 'origin/8.10'.

You are currently cherry-picking commit bc1982aebe.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   changelog/fragments/1695685534-standalone-prevent-quick-upgrades.yaml
	modified:   internal/pkg/agent/application/upgrade/upgrade.go
	modified:   internal/pkg/agent/application/upgrade/upgrade_test.go
	modified:   internal/pkg/agent/cmd/upgrade.go
	modified:   pkg/control/v2/client/client.go
	new file:   pkg/control/v2/client/mocks/client.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   sonar-project.properties
	both modified:   testing/integration/upgrade_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[elasticmachine]

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• Start Time: 2023-10-02T18:14:53.455+0000

• Duration: 6 min 49 sec

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @ycombinator? 🙏

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.10/pr-3473 upstream/mergify/bp/8.10/pr-3473
git merge upstream/8.10
git push upstream mergify/bp/8.10/pr-3473

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @ycombinator? 🙏

[mergify]

This pull request has not been merged yet. Could you please review and merge it @ycombinator? 🙏

[mergify]

This pull request has not been merged yet. Could you please review and merge it @ycombinator? 🙏

[mergify]

This pull request has not been merged yet. Could you please review and merge it @ycombinator? 🙏

[cmacknz]

No more planned 8.10.x releases, closing.