Bump Flatpak runtime to 43

[KamilaBorowska]

Matches flathub/com.github.Alcaro.Flips#3.

[Alcaro]

@Kekun Does this file need to exist in this repo? Isn't the one in the linked repo enough?

[Kekun]

@Alcaro I planned to submit it to you but then forgot, sorry, I'll make a PR ASAP. I may be wrong but I believe Fathub's stable branch requires the modules to be stable targets (commits, tarballs…) and not branches, so usually when publishing a new stable version on Flathub the repos are "pinned" to the relevant commit, but upstream projects tend to retain the moving targets (e.g. master Sdk, main project branch…) as it helps development and gives a clearer indication to where the commits to pin are.

[Alcaro]

Being a hint on what to pin the flatpak downstream stuff to is a valid reason to keep it.

Also superseded by #84, farewell and thanks for the fish.