This avoids warnings about the actions targeting Node 20.
There is no easily available documentation about whether there are
any other API changes in the newer versions of these actions - in
particular the bump of build-push-action which advances by two
versions.
checkout@v5 should be the same as checkout@v4, just running on
Node 24. checkout@v6 has got a few other minor changes that
shouldn't make any difference; opt in to newer behaviours now.