.github: update actions to Node.js 24

All workflow runs now warn:

    Node.js 20 is deprecated. The following actions target Node.js 20
    but are being forced to run on Node.js 24.

Update the actions/* dependencies to their current major versions,
all of which target Node.js 24: checkout v7, upload-artifact v7,
download-artifact v8, setup-python v7, and cache v6.  The release
action tracks the floating v1 tag and updates on its own.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/docs.yml

@@ -30,7 +30,7 @@ jobs:
       version: ${{ steps.version.outputs.version }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
         with:
           fetch-depth: 0  # Needed for git-revision-date-localized plugin
 
@@ -42,7 +42,7 @@ jobs:
           echo "version=$version" >> $GITHUB_OUTPUT
 
       - name: Setup Python
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v7
         with:
           python-version: '3.x'
 
@@ -58,7 +58,7 @@ jobs:
         run: mkdocs build --clean
 
       - name: Upload site artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: site
           path: site/
@@ -72,13 +72,13 @@ jobs:
 
     steps:
       - name: Download site artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v8
         with:
           name: site
           path: site/
 
       - name: Checkout pages repo
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
         with:
           repository: finit-project/finit-project.github.io
           path: pages