rust: pin-init: internal: turn `PhantomPinned` error into warnings

rust: pin-init: internal: turn `PhantomPinned` error into warnings

The `PhantomPinned` detection is just a lint, and is emitted as an error
because there is no `compile_warning!()` macro, and
`proc-macro-diagnostics` is not stable.

Use of `#[deprecated = ""]` attribute to approximate custom proc-macro
warnings. A new line is added before message for visual clarity.

An example warning with this trick looks like this:

    warning: use of deprecated function `_::warn`:
             The field `pin` of type `PhantomPinned` only has an effect if it has the `#[pin]` attribute
     --> test.rs:9:5
      |
    9 |     pin: marker::PhantomPinned,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/pin-init/issues/51
Link: https://patch.msgid.link/20260428-pin-init-sync-v1-10-07f9bd3859fb@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>