Fix MessageSnapshot sticker_items

[vionya]

Summary

MessageSnapshots were creating their list of sticker items from the stickers_items attribute which does not exist in the incoming data (the correct attribute is called sticker_items), and as such message snapshots did not properly reflect the stickers contained in the forwarded message.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes. (N/A)
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)