Document minimum required Qt version

[hebasto]

In the current state of this repo it is supposed that the bitcoin-qt binary could be built against dynamically linked Qt packages provided by a system package manager (apt -- on Debian-based systems, Homebrew -- on macOS).

The current Qt version on Homebrew is 5.15.2.

Qt versions on some Linux distros:

• buster -- 5.11.3
• bullseye -- 5.15.2
• focal LTS, supported until 2025-04 -- 5.12.8
• hirsute -- 5.15.2

The current Qt version in depends is:

gui-qml/depends/packages/qt.mk

Line 2 in 955fa34

$(package)_version=5.12.11

From Qt blog:

• https://www.qt.io/blog/2019/01/11/qt-5-12-lts-road-faster-qml-application-startup
• https://www.qt.io/blog/qml-type-registration-in-qt-5.15
• https://www.qt.io/blog/new-qml-language-features-in-qt-5.15

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My suggestion is to stick to Qt 5.12 LTS that allows developers and testers use a broader set of Linux systems, unless Qt 5.15 has crucial for this project features.