main/postmarketos-release-upgrade: upgrade to 1.6.1

Contains this fix: https://gitlab.com/postmarketOS/postmarketos-release-upgrade/-/merge_requests/16

Let's backport it to v23.06 and v23.12. For future releases, we shouldn't put it into the previous release before the new one is released - but given that 1.6.0 is also in v23.06 already, let's backport this as well.