Automated submodule update: vrs (#353)

Summary:
This is an automated pull request to update the first-party submodule for [facebookresearch/vrs](https://github.com/facebookresearch/vrs).

New submodule commit: facebookresearch/vrs@6c647a7

Pull Request resolved: #353

Test Plan: Ensure that CI jobs succeed on GitHub before landing.

Reviewed By: georges-berenger

Differential Revision: D95288810

fbshipit-source-id: 6ba8e48c5e2780198cf10edd633de62a9662bde3

Author: facebook-github-bot