externals: Revert SDL2 to release-2.0.20

prerelease-2.23.1 appears to have issues on the SteamDeck with external
controllers. Revert to 2.0.20 for now (and as opposed to using
prerelease-2.0.19 like before.)

externals/SDL

@@ -1 +1 @@
-Subproject commit 59fb7acbf7af9d64a2d5432bb6677585a0ddd50a
+Subproject commit b424665e0899769b200231ba943353a5fee1b6b6