Qt6 as the default also in S3_1
Qt6 has gone in to the Debian Unstable - expected to migrate to testing in a couple of days. Also Ubuntu has picked that immediately, so it seems that Ubuntu-22.04 LTS will have it after all. With most current systems then having Qt6, we should consider making it the default also in S3_1 already (I think we definitely want to make it the default before releasing 3.1.0, which is still years away)
Before this, I'd like #43958 to be in (doing it the other way around may cause some people to note the "Qt6 requirement", and to never look at it again)
Change to S3_1 attached, and also a patch for master that changes README.packaging accordingly.
- Rebased master patch to apply on top of (more urgent) #44018