[ty] Disable possibly-missing-imports by default (#22041)

@carljm put forth a reasonably compelling argument that just disabling
this lint might be advisable. If we agree, here's the implementation.

* Fixes https://github.com/astral-sh/ty/issues/309

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/src/types/diagnostic.rs

@@ -1594,7 +1594,7 @@ declare_lint! {
     pub(crate) static POSSIBLY_MISSING_IMPORT = {
         summary: "detects possibly missing imports",
         status: LintStatus::stable("0.0.1-alpha.22"),
-        default_level: Level::Warn,
+        default_level: Level::Ignore,
     }
 }