feat: ✨ Parse data as message in parse_message_update instead of using _update

[Paillat-dev]

Summary

Superseeds: #2778 - no need to merge both.
Fixes #2777
Message update contains a full message object - no need to use that messy update logic. See:

• https://discord.com/developers/docs/events/gateway-events#message-update
• https://discord.com/channels/613425648685547541/697138785317814292/1273694894036353038

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

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[Soheab]

Not a fan of the attribute name new_message, but it makes sense and is fine for now. Good stuff.

meow