Qt 6.5 as the only supported Qt version

[TrickyLeifa]

Given our dependency on Qt and the constant evolving state of the framework, stabilizing on one version would be ideal as it would reduce potential problems when trying to build against older or newer versions.

As of writing, Qt 6.5 is the current LTS version and I propose we stick to it for the time being. Newer LTS versions can always be discussed and considered in the future, but the codebase needs to avoid having to cover potentially everything.

Obviously, this implies that under several distros we can no longer rely on the typical package system since a lot of them won't have the latest LTS for a while. In that case, an alternative like AppImage or Flatpak would provide the necessary workaround to allow Linux users to still use the client without requiring to tell them to manually install all manner of different libraries.

[TrickyLeifa]

#991 solves the versioning.

A future pull request is required to generate AppImage for Linux builds, otherwise they most likely won't be able to use the client for a couple years unless they run bleeding edge.