[8.x](backport #3142) [Asset Inventory][Azure] Do not add --install-servers flag for versions < 9.0

[mergify]

Summary of your changes

Fixes an issue where releases earlier than 9.0 also got an --install-servers flag. Using the same code as regular Azure install script.

Related Issues

Fixes https://github.com/elastic/security-team/issues/11500

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This is an automatic backport of pull request #3142 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of cc1ed67 has failed:

On branch mergify/bp/8.x/pr-3142
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit cc1ed67b.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deploy/asset-inventory-arm/install-agent.sh

no changes added to commit (use "git add" and/or "git commit -a")

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

[mergify]

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