Fix some problems I encountered

[gbarranco]

Hello, I encountered 2 kind of problems when using your roles, so I thought I'll make a PR to suggest you some changes. The first one is, there wasn't become: true on your task to escalate privileges, so I had to manually put the password in Gnome popups many times and in the end I had an error anyway (the password was right and copy / pasted each time).

The second one was when uninstalling snap, there was an error on some firefox files that couldn't be deleted because the filesystem on which they were was mounted in read-only. This is why I added the task to unmount it (the two states unmounted and absent are present because it was the only effective way to unmount the volume).

I also had a deprecation warning on the task uninstall snaps in the uninstall snaps.yml file because of the community.general.snap that seems to use CmdMixin that is deprecated. But I didn't know how to fix it.

Also, after all that, I would like to thank you for your role, it was helpful for me and made me save a lot of time :)