Doc: Add 9.1.6 release notes noting no user-facing changes

[karenzone]

Manual update to release notes indicating that there are no user-facing changes for 9.1.6

Related: Fleet and Agent 9.1.6 release notes

[prodsecmachine]

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[mergify]

This pull request does not have a backport label. Could you fix it @karenzone? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[karenzone]

@colleenmcginnis, questions.

• Is this the right approach for adding a "no-user-facing-changes" update to release notes?
• I based this PR on main. Usually the team works out of the release branch and FPs to main. It seems to make sense to backport to 9.1 for completeness. Please let me know what you think.