ci: Run tests against Zephyr v3.7 branch

This commit updates the CI workflow in the SDK 0.16 LTS branch to run
tests against the Zephyr v3.7 LTS3 branch.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

.github/workflows/ci.yml

@@ -88,7 +88,7 @@ env:
   BUG_URL: 'https://github.com/zephyrproject-rtos/sdk-ng/issues'
   BUNDLE_NAME: Zephyr SDK
   BUNDLE_PREFIX: zephyr-sdk
-  ZEPHYR_REF: collab-sdk-dev
+  ZEPHYR_REF: v3.7-branch
 
 jobs:
   # Setup