Update to actions/checkout@v4.

Some deprecation warnings from GitHub seem to indicate that we should do
that.

.github/workflows/archive.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: "Checkout"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: "Update Archive"
       uses: martinthomson/i-d-template@v1

.github/workflows/ghpages.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: "Checkout"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: "Cache Setup"
       id: cache-setup

.github/workflows/publish.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: "Checkout"
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     # See https://github.com/actions/checkout/issues/290
     - name: "Get Tag Annotations"