fix: revert to weakrefs for user cache

[shiftinv]

Summary

This largely reverts cb2363f.

Users were not being evicted as expected, resulting in objects being retained solely in the user cache, with no other strong references.
See https://docs.python.org/3.10/library/weakref.html#weakref.WeakValueDictionary and https://docs.python.org/3.10/reference/datamodel.html#object.del.
The difference in performance and memory usage is insignificant, at least in 3.10; haven't tested other versions, but this is a bug regardless of version.

To reproduce, anything that uses state.store_user can be used, for example the GUILD_BAN_REMOVE handler:

print(bot.get_user(21414249976823808))
await ctx.guild.ban(disnake.Object(21414249976823808))
await ctx.guild.unban(disnake.Object(21414249976823808))
await asyncio.sleep(0.5)
print(bot.get_user(21414249976823808))

With the previous implementation, the second print would output the user (assuming members intent is enabled) - with the changes in this PR, both print None as expected.

(for context, this was originally noticed a while ago in an internal channel here)

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 task lint
  • I have type-checked the code by running task 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, ...)