ContextualMenu: Fix Narrator Separator Announcement

[JasonGore]

Pull request checklist

• Addresses an existing issue: Fixes Narrator announces the ContextualMenu separator #6848, ContextualMenu A11y Pass #6360
• Include a change request file using $ npm run change

Description of changes

Add aria-hidden attribute to separator to prevent announcement in Narrator's scan mode.

Microsoft Reviewers: Open in CodeFlow

[codecov-io]

Codecov Report

Merging #6876 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6876   +/-   ##
=======================================
  Coverage   74.84%   74.84%           
=======================================
  Files         858      858           
  Lines       24223    24223           
  Branches     5319     5319           
=======================================
  Hits        18130    18130           
  Misses       5646     5646           
  Partials      447      447

Impacted Files	Coverage Δ
.../components/ContextualMenu/ContextualMenu.base.tsx	74.4% <ø> (ø)	⬆️

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[msft-github-bot]

🎉office-ui-fabric-react@v6.93.1 has been released which incorporates this pull request.:tada:

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