Update RHEL9 docs only to 2.15.6

[t-mialve]

Update only the RHEL9 docs to 2.15.6 (staggered rollout, other distros to follow in January)

[prmerger-automator]

@t-mialve : Thanks for your contribution! The author(s) have been notified to review your proposed change.

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Learn Build status updates of commit fff2ba0:

✅ Validation status: passed

File	Status	Preview URL	Details
azure-managed-lustre/client-install.md	✅Succeeded
azure-managed-lustre/includes/client-install-version-rhel-8-9.md	✅Succeeded	n/a (file deleted or renamed)
azure-managed-lustre/includes/client-install-version-rhel-8.md	✅Succeeded
azure-managed-lustre/includes/client-install-version-rhel-9.md	✅Succeeded

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PRMerger Results

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[Jak-MS]

@t-mialve - when you're ready for this to be reviewed, can you take it out of draft mode? thanks!

@pauljewellmsft
Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team