Switch to v3 of checkout action

v2 seems to still use Node.js 12, which is deprecated and causes lots of
annotations/warnings in our workflows.

.github/workflows/build-and-test.yml

@@ -12,7 +12,7 @@ jobs:
         python-version: [3.7, 3.8, 3.9]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: "Cache pip packages"
         uses: actions/cache@v3
@@ -132,7 +132,7 @@ jobs:
     name: "Test Javascript code"
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: browser-actions/setup-chrome@latest
       - uses: browser-actions/setup-firefox@latest
       - run: chrome --version

.github/workflows/dockercompose.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: docker-compose build
         run: |

.github/workflows/linter.yml

@@ -39,7 +39,7 @@ jobs:
       # Checkout the code base #
       ##########################
       - name: Checkout Code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           # Full git history is needed to get a proper list of changed files within `super-linter`
           fetch-depth: 0