Replace DNF's deprecated 'dnf update' alias with 'dnf upgrade'

[JWShaw]

Addresses #1298. According to the DNF manpages, update is a deprecated alias.

[ldennington]

Thank you for your contribution! To better understand the scope of this change, can you provide the version of RHEL/Fedora in which the update command was initially deprecated?

[JWShaw]

I've not been successful in finding the exact version of RHEL/Fedora in which update was initially deprecated, but I found by perusing the dnf repository that dnf update was deprecated by a commit on November 12, 2012.

[ldennington]

I've not been successful in finding the exact version of RHEL/Fedora in which update was initially deprecated, but I found by perusing the dnf repository that dnf update was deprecated by a commit on November 12, 2012.

Thanks for doing this digging! I think we're good since the command was deprecated over 10 years ago, and none of the versions of the applicable distros supported by .NET were released before that time. I've also done a test run of our validation workflow against this branch, and it's all green! I'll go ahead and get this merged.