Qt 6.5 as the default 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.