gh-actions: move actions/checkout to main

.github/workflows/check-pr.yml

@@ -21,7 +21,7 @@ jobs:
       matrix:
         check: [commit-msg, pr_check]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
       with:
         # Use the SHA of the PR branch as-is, not the PR branch merged
         # in master (default behavior in GH actions)

.github/workflows/release-test.yml

@@ -88,14 +88,14 @@ jobs:
         ssh -oStrictHostKeyChecking=accept-new \
           ${IOTLAB_USER}@lille.iot-lab.info exit
     - name: Checkout Release-Specs
-      uses: actions/checkout@v2
+      uses: actions/checkout@main
       with:
         repository: RIOT-OS/Release-Specs
         path: Release-Specs
         fetch-depth: 1
         ref: ${{ github.event.inputs.release_specs_version }}
     - name: Checkout RIOT
-      uses: actions/checkout@v2
+      uses: actions/checkout@main
       with:
         repository: RIOT-OS/RIOT
         path: RIOT

.github/workflows/static-test.yml

@@ -16,7 +16,7 @@ jobs:
   static-tests:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@main
       with:
         fetch-depth: 0
     - name: Setup git

.github/workflows/test-on-iotlab.yml

@@ -110,7 +110,7 @@ jobs:
         run: |
           docker pull riot/riotbuild:latest
       - name: Checkout RIOT
-        uses: actions/checkout@v2
+        uses: actions/checkout@main
         with:
           ref: ${{ github.event.inputs.riot_version }}
       - name: Launch IoT-LAB experiment

.github/workflows/test-on-ryot.yml

@@ -85,7 +85,7 @@ jobs:
       APPLICATIONS_EXCLUDE: 'tests/periph_timer_short_relative_set'
     steps:
       - name: Checkout RIOT
-        uses: actions/checkout@v2
+        uses: actions/checkout@main
         with:
           ref: ${{ github.event.inputs.riot_version }}
           # Make sure it runs git clean -xdff before fetching

.github/workflows/tools-buildtest.yml

@@ -14,7 +14,7 @@ jobs:
   tools-build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - name: Build flatc standalone
       uses: aabadie/riot-action@v1
       with:

.github/workflows/tools-test.yml

@@ -14,7 +14,7 @@ jobs:
   python-tests:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@main
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v1
       with: