### What?
There were a couple issues with the implementation within the example
when using postgres.
- `ensureUniqueUsername` tenant was being extracted incorrectly, should
not constrain query unless it was present
- `ensureUniqueSlug` was querying by NaN when tenant was not present on
data or originalDoc
- `users` read access was not correctly extracting the tenant id in the
correct type depending on DB
Fixes https://github.com/payloadcms/payload/issues/11484
- Adds optional tenant-based cookie handling based by domain (commented
out to leave functionality out by default)
- Removes 2.0 multi-tenant example
- Updates `examples/multi-tenant-single-domain` -->
`examples/multi-tenant`
