gitmodules: use testing-devel branch for fedora-coreos-config

Also bump submodule to HEAD of testing-devel

```
$ git shortlog --no-merges 6f39113b0bd0e40d783edcfedc69b93b0492b096..43eb8766112420a2a24d3f872da088bbd752aa48
Andy McCrae (1):
      Move comments in systemd services to own line

Colin Walters (3):
      coreos-boot-mount-generator: Move all "exit 0" checks to early on
      udev/90-coreos-device-mapper: Create label links in real root too
      coreos-boot-mount-generator: Always use mpath for /boot if rd.multipath

Dusty Mabe (1):
      overlay: initramfs teardown: add support for coreos.force_persist_ip karg

Jonathan Lebon (10):
      ignition-ostree-growfs: don't conditionalize on root= karg
      ignition-ostree-growfs: fix dependency to sysroot mount
      overlay: convert more unit descriptions to Title Case
      coreos-gpt-setup: log whether sgdisk was called
      ignition-ostree-uuid-root: also run if root= is provided
      live-generator: don't call `man bootup`
      Add support for multipath on firstboot
      coreos-gpt-setup: add support for multipath
      ignition-ostree-growfs: add support for multipath
      udev/90-coreos-device-mapper: ignore DM_ACTIVATION
```

Author: miabbott

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "fedora-coreos-config"]
 	path = fedora-coreos-config
 	url = https://github.com/coreos/fedora-coreos-config
-	branch = rhcos-4.8
+	branch = testing-devel