Update checkout action to v3.

Author: hageboeck

.github/workflows/build-exercises.yml

@@ -97,7 +97,7 @@ jobs:
     # The build/test steps to execute.
     steps:
     # Use a standard checkout of the code.
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     # Make sure that Python 3 is available to the build.
     - uses: actions/setup-python@v2
       with:

.github/workflows/build-slides.yml

@@ -21,7 +21,7 @@ jobs:
         version: [ essentials, full ]
     steps:
       - name: Set up Git repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Set up essentials course
         if: matrix.version == 'essentials'
         run: echo '\basictrue' > talk/onlybasics.tex

.github/workflows/publish-slides.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         version: [ essentials, full ]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Setup essentials course
         if: matrix.version == 'essentials'
         run: echo '\basictrue' > talk/onlybasics.tex