build: Switch to Qt 6
The currently used Qt 5.15 is approaching EOL. The recent migration of the Bitcoin Core's build system to CMake makes it possible to switch to Qt 6.
This PR updates the OS runtime compatibility requirements for the Bitcoin Core GUI as follows:
1. Linux
Starting with Qt 6.5.0, the libxcb-cursor0 package is required to be installed at runtime.
2. Windows
Qt 6 requires Windows 10 (1809 or later) / Windows 11 (also see https://github.com/bitcoin/bitcoin/pull/31172). The new code expects Qt 6.7 or newer, which holds automatically for cross-compiled builds. A new style plugin causes minor visual glitches that will be fixed in follow-ups.
3. macOS
bitcoin-qt now uses the Metal backend.
IMPORTANT. Don't forget to install Ninja.
For historical context, please refer to:
UPD 2024-10-09. Qt 6.8 has been released, but it has some drawbacks for us. As a result, this PR will stick to Qt 6.7.