stubgen: use PEP 604 unions everywhere

[hamdanal]

Fixes #12920

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

parso (https://github.com/davidhalter/parso): typechecking got 16796.73x faster (10.4s -> 0.0s)
(Performance measurements are based on a single noisy sample)

anyio (https://github.com/agronholm/anyio): typechecking got 3.35x faster (16.7s -> 5.0s)
(Performance measurements are based on a single noisy sample)

discord.py (https://github.com/Rapptz/discord.py): typechecking got 1.07x slower (176.3s -> 189.2s)
(Performance measurements are based on a single noisy sample)

[hamdanal]

typechecking got 16796.73x faster (10.4s -> 0.0s)

[hamdanal]

Thanks @JelleZijlstra. Seems like a test was added after I created the PR https://github.com/python/mypy/actions/runs/7513432949/job/20455178894 and it fails. I'll send a fix.