feat: add silent messages to MessageFlags

[JustaSqu1d]

Summary

discord/discord-api-docs#5910

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.

[JustaSqu1d]

I don't have Experiments. Could someone test this?

[codecov]

Codecov Report

Merging #1912 (ee826c2) into master (546f051) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1912      +/-   ##
==========================================
+ Coverage   33.32%   33.33%   +0.01%     
==========================================
  Files          97       97              
  Lines       18750    18753       +3     
==========================================
+ Hits         6249     6252       +3     
  Misses      12501    12501              

Flag	Coverage Δ
macos-latest-3.10	33.32% <100.00%> (+0.01%)	⬆️
macos-latest-3.11	33.32% <100.00%> (+0.01%)	⬆️
macos-latest-3.8	33.33% <100.00%> (+0.01%)	⬆️
macos-latest-3.9	33.33% <100.00%> (+0.01%)	⬆️
ubuntu-latest-3.10	33.32% <100.00%> (+0.01%)	⬆️
ubuntu-latest-3.11	33.32% <100.00%> (+0.01%)	⬆️
ubuntu-latest-3.8	33.33% <100.00%> (+0.01%)	⬆️
ubuntu-latest-3.9	33.33% <100.00%> (+0.01%)	⬆️
windows-latest-3.10	33.32% <100.00%> (+0.01%)	⬆️
windows-latest-3.11	33.32% <100.00%> (+0.01%)	⬆️
windows-latest-3.8	33.33% <100.00%> (+0.01%)	⬆️
windows-latest-3.9	33.33% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
discord/flags.py	76.09% <100.00%> (+0.17%)	⬆️

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[Lulalaby]

I don't have Experiments. Could someone test this?

This is rolled out to all users on all versions (Update if possible, I won't search all version numbers rn)

[JustaSqu1d]

I don't have Experiments. Could someone test this?

This is rolled out to all users on all versions (Update if possible, I won't search all version numbers rn)

Tested. It works.