Update actions/checkout to v3 (#1074)

This fixes warnings about node 12 actions being deprecated.

Author: alcaeus

.github/workflows/coding-standards.yml

@@ -30,7 +30,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v2"
+        uses: "actions/checkout@v3"
 
       - name: Setup cache environment
         id: extcache

.github/workflows/static-analysis.yml

@@ -30,7 +30,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v2"
+        uses: "actions/checkout@v3"
 
       - name: Setup cache environment
         id: extcache

.github/workflows/tests.yml

@@ -66,7 +66,7 @@ jobs:
 
     steps:
       - name: "Checkout"
-        uses: "actions/checkout@v2"
+        uses: "actions/checkout@v3"
         with:
           fetch-depth: 2