Add @Deprecated annotation to a synthetic $serializer class

in K2 so it matches K1 behavior.

Deprecation is required, so the declaration will not be visible to user in completion in the IDE.

#KT-65757 Fixed

plugins/kotlinx-serialization/testData/diagnostics/GeneratedSerializerInaccessible.kt

@@ -0,0 +1,14 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+// SKIP_TXT
+
+import kotlinx.serialization.*
+
+@Serializable
+class A
+
+fun foo() {
+    A.<!DEPRECATION_ERROR!>`$serializer`<!>
+    A.<!DEPRECATION_ERROR!>`$serializer`<!>.descriptor
+    A(<!TOO_MANY_ARGUMENTS!>0<!>, <!TOO_MANY_ARGUMENTS!>null<!>)
+}