Use HashSet instead of Hashtable with null values

[xtqqczze]

PR Summary

• Replace use non-generic Hashtable
• Dictionary not used as we don't store values.

PR Context

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[iSazonov]

@xtqqczze Please rebase to pass CIs.

[ghost]

🎉v7.1.0-preview.4 has been released which incorporates this pull request.:tada:

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