feat: further username system updates

[shiftinv]

Summary

continuation of #1025, incorporating the new updates from discord/discord-api-docs@6e9622b

Changes:

• Calculate default avatars of migrated users using mod 6 instead of 5, which results in the fuchsia/pink default avatar now being used ^{(woo \o/)}
• Revert the "bot display name = application name" part of the documentation, as Discord has now decided against it
• Clarify other parts of the documentation regarding bot users not getting pomelo'd (for now)

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• 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, ...)