qt: drop support for 5.11
5.11 support was in place to be aligned with debian Buster version. Buster is old stable for quite a while it doesn't feels relevant to keep supporting it. for reference debian Bulleyes ships 5.15, Ubuntu 20.04 ships 5.12 (and 22.04 -> 5.15)
Amongst the perks of using 5.12 as a minimal version:
- Qt JS engine is compatible ES6
- better touch/tap handler support
- 5.12 is a Qt LTS (5.11 wasn't)