Update nRF-Connect SDK 3.1 Stable and GH actions/checkout (#23)

* ci: nRF-Connect SDK 3.1 Stable Branch

* ci: Update actions/checkout

Author: ShineBrightMeow

.github/workflows/workflow.yml

@@ -37,7 +37,7 @@ jobs:
       - name: 🧹 Clean Landing Site
         run: |
           sudo rm -rf zephyr-workspace/SlimeNRF-CI
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           # Clone the repo to a subdirectory, so we can initialize the Zephyr
           # workspace in the parent directory.

west.yml

@@ -6,5 +6,5 @@ manifest:
     - name: sdk-nrf
       path: nrf
       url: https://github.com/nrfconnect/sdk-nrf.git
-      revision: v3.1.0-preview4-branch
+      revision: v3.1-branch
       import: true