fix: Adjust default_avatar behavior depending on username

[NeloBlivion]

Summary

As per discord/discord-api-docs#6130 (comment), default_avatar should still use discrim if the user hasn't migrated yet.

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.

[codecov]

Codecov Report

Merging #2087 (99d3134) into master (7bbb42a) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2087      +/-   ##
==========================================
- Coverage   33.15%   33.14%   -0.01%     
==========================================
  Files          97       97              
  Lines       19125    19126       +1     
==========================================
  Hits         6340     6340              
- Misses      12785    12786       +1     

Flag	Coverage Δ
macos-latest-3.10	33.13% <0.00%> (-0.01%)	⬇️
macos-latest-3.11	33.13% <0.00%> (-0.01%)	⬇️
macos-latest-3.8	33.14% <0.00%> (-0.01%)	⬇️
macos-latest-3.9	33.14% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.10	33.13% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.11	33.13% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.8	33.14% <0.00%> (-0.01%)	⬇️
ubuntu-latest-3.9	33.14% <0.00%> (-0.01%)	⬇️
windows-latest-3.10	33.13% <0.00%> (-0.01%)	⬇️
windows-latest-3.11	33.13% <0.00%> (-0.01%)	⬇️
windows-latest-3.8	33.14% <0.00%> (-0.01%)	⬇️
windows-latest-3.9	33.14% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
discord/user.py	37.50% <0.00%> (-0.19%)	⬇️

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