Backport "Nopos-warn in vulpix & Move warn tests form tests/neg to tests/warn: Batch 1" to LTS (#20821)

Backports #19242 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

tests/neg-deep-subtype/i3324.scala

@@ -0,0 +1,5 @@
+-- Error: tests/neg/i11225b.scala:6:26 ---------------------------------------------------------------------------------
+6 |  private var cached: A = _ // error
+  |                          ^
+  |                          `= _` has been deprecated; use `= uninitialized` instead.
+  |                          `uninitialized` can be imported with `scala.compiletime.uninitialized`.

tests/neg/i10247.scala

@@ -10,3 +10,4 @@ class Memo[A](x: => A):
       known = true
       cached = x
     cached
+