Warning with 2.0.5.109 is under investigation as a regression from 2.0.5.108. Early test shows it affects Windows users using VSS snapshot-policy to backup drive root (e.g. C:\), basically not backing that up.
Those doing such backup might want to hold off update. If you update and error, follow up at issue topic.
Users who are not on Windows should use mono 5.10 or above. Previously any version 5 would do.
Most distributions that went to mono v5 at all should be fine. You can run mono --version to check. https://repology.org/project/mono/versions gives a nice compilation of versions for distros.
Windows is quite good at updating .NET Framework, so the move to 4.7.1 from 4.6.2 should be fine.