Since Mill is on Scala 3, it's time for the examples to be Scala 3 as
well. Also bumped library versions in the examples

Left a few on Scala 2 where the upgrade was a bit fiddly (e.g. dealing
with fatal warnings, compiler plugins, etc.)

…m-lihaoyi#5404)

This reverts commit d03fd45.

Fixes com-lihaoyi#5398

fixes com-lihaoyi#5403

- `mill.vcs` needs to be stable
- `mill.tabcomplete` does not need to be stable
- `mill.scalanativelib`, `mill.scalajslib`, `mill.kotlinlib` can
probably be stable by now

…om-lihaoyi#5407)

- `core/api/` has been moved to `core/api/shared/`, under
`mill.api.shared`
- `core/define/`  has been moved to `core/api/`, under `mill.api`
- `mill.api` now has explicit exports for the relevant user-facing
classes in `mill.api.shared`

The goal of this is to eliminate the user-facing split between
`mill.define` and `mill.api`. Previously, the split was purely based on
the internal concerns of what classes are necessary to interface with
`mill.daemon`, and is totally irrelevant to the user. This causes
confusion in where to find things, since it's generally impossible to
remember what lives in which one

This change unifies the user-facing API under `mill.api`, while keeping
the backend split of `core/api` and `core/api/shared` to accommodate the
separation necessary internally. Now users can directly import stuff
from `mill.api` all the time, and whether the actual implementation
lives in `mill.api` or `mill.api.shared` doesn't really matter since
everything that lives elsewhere is re-exported

For now the number of things exported from `import mill.api._` and
`import mill._` is somewhat arbitrary, but we can add more exports later
if necessary