chore(workflows): Update checkout action to v3 (#175)

This change was done as the v2 version still used the node v12 runtime.
This runtime is deprecated by github. There are no other breaking
changes done by the action. With the update to v3 now node v16 is used
as runtime.

You can read more about the changes from v2 to v3 here:
https://github.com/actions/checkout/releases/tag/v3.0.0

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
 
     steps:
     - name: Checkout sources
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
     - name: Install toolchain
       uses: actions-rs/toolchain@v1
       with:
@@ -65,7 +65,7 @@ jobs:
 
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install toolchain
         id: tc
@@ -127,7 +127,7 @@ jobs:
 
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install toolchain
         id: tc
@@ -162,7 +162,7 @@ jobs:
 
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install toolchain
         id: tc

.github/workflows/docs.yml

@@ -13,7 +13,7 @@ jobs:
 
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install toolchain
         id: tc