[8.10](backport #3384) Uninstall finds and kills any running elastic-agent watch process

[mergify]

This is an automatic backport of pull request #3384 done by Mergify.
Cherry-pick of 7e86d24 has failed:

On branch mergify/bp/8.10/pr-3384
Your branch is up to date with 'origin/8.10'.

You are currently cherry-picking commit 7e86d24cd3.
  (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/1694187216-Uninstall-finds-and-kills-any-running-watcher-process.yaml
	modified:   internal/pkg/agent/cmd/uninstall.go
	modified:   internal/pkg/agent/install/uninstall.go
	new file:   pkg/utils/watcher.go
	modified:   testing/integration/upgrade_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   internal/pkg/agent/cmd/install.go
	both modified:   internal/pkg/agent/install/install.go
	deleted by us:   internal/pkg/agent/install/progress.go
	deleted by us:   internal/pkg/agent/install/progress_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]

💔 Build Failed

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• Start Time: 2023-10-02T19:58:19.911+0000

• Duration: 5 min 14 sec

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• Took 0 min 11 sec . View more details here
• Description: make check-ci

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[blakerouse]

This cannot be backported.