migration-guide: add release notes for 4.2.4

release notes based on 4.2.4 rc3.

(From yocto-docs rev: de4fb1f877ae9994c61dfc60447446da317bee0f)

Signed-off-by: Lee Chee Yang <chee.yang.lee@intel.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/release-4.2.rst

@@ -10,3 +10,4 @@ Release 4.2 (mickledore)
    release-notes-4.2.1
    release-notes-4.2.2
    release-notes-4.2.3
+   release-notes-4.2.4