doc: release: Deprecate CONFIG_MULTITHREADING=n in 2.4.0 release notes

Adds a note about deprecating single-threaded mode to the 2.4.0 release
notes.

Signed-off-by: Maureen Helm <[email protected]>

Author: MaureenHelm

doc/releases/release-notes-2.4.rst

@@ -134,6 +134,11 @@ Deprecated in this release
   * ``sys_mem_pool_free()``
   * ``sys_mem_pool_try_expand_inplace()``
 
+* The Kconfig option ``CONFIG_MULTITHREADING`` to disable multi-threading was
+  deprecated due to lack of maintainership. This means that single-threaded
+  mode with the scheduler disabled is deprecated; normal multi-threaded mode is
+  still fully supported.
+
 Removed APIs in this release
 ============================