ZTS: Update deprecated Github Action version numbers

GitHub Actions is transitioning from Node 16 to Node 20.

So we need to update these:
- actions/checkout@v3 -> v4
- actions/download-artifact@v3 -> v4
- actions/upload-artifact@v3 -> v4 and some minor changes

Update also the documentation of the testings workflow.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Andrew Innes <andrew.c12@gmail.com>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #15820

.github/workflows/checkstyle.yaml

@@ -8,7 +8,7 @@ jobs:
   checkstyle:
     runs-on: ubuntu-22.04
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
     - name: Install dependencies
@@ -52,7 +52,7 @@ jobs:
       if: failure() && steps.CheckABI.outcome == 'failure'
       run: |
         find -name *.abi | tar -cf abi_files.tar -T -
-    - uses: actions/upload-artifact@v3
+    - uses: actions/upload-artifact@v4
       if: failure() && steps.CheckABI.outcome == 'failure'
       with:
         name: New ABI files (use only if you're sure about interface changes)