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/scripts/generate-summary.sh

@@ -87,7 +87,7 @@ function summarize_f() {
   output "\n## $headline\n"
   rm -rf testfiles
   for i in $(seq 1 $FUNCTIONAL_PARTS); do
-    tarfile="$2/part$i.tar"
+    tarfile="$2-part$i/part$i.tar"
     check_tarfile "$tarfile"
     check_logfile "testfiles/log"
   done