Replace yum module with dnf

[aeneby]

Not sure if this has any practical implications, but since ThinLinc 4.19 we don't support YUM anymore. We should update this role accordingly.

[aeneby]

Although there are still supported versions of ThinLinc with the YUM backend, there aren't any supported versions of Ansible with it; using yum just redirects to the dnf module.

So strictly speaking, we would need to keep using the yum module in order for people to install a supported version of ThinLinc on an EOL distribution, using an EOL version of Ansible. Not something we want to encourage perhaps, but since this has no practical implications for those wanting to install on dnf systems using modern Ansible, we can probably leave it until there are no more supported ThinLinc versions with the YUM backend anymore (December 2027).