A couple of changes to support Fedora

[AdamWill]

This adds a Fedora block to install_prereq.sh, and enhances start_flash.sh to handle disabling systemd-resolved at the start and re-enabling it at the end, if it's being used (it listens on port 53, which this tool needs to take over).

In the Fedora block we use a PR branch of sslpsk which makes it work with Python 3.12 and later, because current Fedoras are on 3.12 or 3.13. Other distros may also need this change now or soon. See https://github.com/drbild/sslpsk/pull/28 .

This should make it work more or less out of the box on Fedora, following the instructions. I was able to convert a couple of old smart plugs I found lying around after doing the equivalent of these changes, so thanks for the tool!