refactor: remove ibis.NA

[ncclementi]

Opening an issue to discuss a separate the refactor of ibis.NA because doing it as part of #9300 will get messy.

To discuss:
Does removing ibis.NAimply:

1. Replace ibis.NA for ibis.null()
2. Get rid of untyped nulls all together, as in not use ibis.null() but just None

[jcrist]

Another option is to rename it to ibis.NULL, but I'd (slightly) vote for option 1.

I'm not really sure what option 2 means - in many cases APIs that accept None should implicitly coerce it to a literal NULL, but not in all cases. There will always cases where calling ibis.null() (or ibis.literal(None), which is the same thing) will be necessary or more ergonomic.

[cpcloud]

I don't think can we remove ibis.null(), it accepts a dtype and can create a typed null which is necessary and/or useful in some cases.

I would vote for the following:

1. Remove NA
2. Don't add NULL
3. Use None wherever untyped NULLs previously worked.

ibis.null(...) remains unchanged.

[cpcloud]

As far implementation goes, we can basically search and replace ibis.NA with ibis.null(). If folks want an untyped NULL, that's None (after these changes).

[ncclementi]

For implementation purposes, do we want to deprecate ibis.NAor remove it as a breaking change. If it's a deprecation, how do we deprecate ibis.NA = null() the @deprecated seems to only work for methods/functions.