Use floodgate for ratelimiting

[Lunarmagpie]

I tested it and im pretty confident it works

[Lunarmagpie]

I think this is blocked until hikari is updated to the newest version, or the version needs to be locked.

[FuexFollets]

May I ask what the purpose is of using floodgate?

[Lunarmagpie]

May I ask what the purpose is of using floodgate?

I want to make this change for 2 reason:

• The old library pycooldown is being deprecated in favor of this one.
• I think the API is much better.

Floodgate is also faster than pycooldown but that doesn't matter that much.

[Lunarmagpie]

I merged this with upstream—CI passes so it should be reviewed