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