Update optional and union syntax in docs and code

[ToothyDev]

Currently, both the docs and old code uses both the old style Union[...] and Optional[...] as well as the new style ... | ... and ... | None.

Given the new style is supported in all Python versions that Pycord currently supports, we should unify the syntax we use to use the new style; It also arguably makes them easier to read, particularly for large unions.

Scope is both all library code and all library docs, so this might be a large piece of repetitive work

[Icebluewolf]

Whoever tackles this should also review all open PRs to ensure that all pending code uses the unified syntax