Provide API to define qsb version in QShaderBaker

Currently baked QShaders support is forward-compatible, so shaders generated with Qt 6.x are compatible with that version and newer. This works when shaders are baked at runtime and/or shaders are used with the same Qt where the QShaderBaker is from.

But in future it may come more relevant that shaders are baked with QShaderBaker from newer Qt, while user wants to use shaders with older (LTS) Qt. So there could be API to set the version manually to older "compatibility" version.

Support should start from version 6 (Qt 6.3+) or 5 (Qt 6.0 - 6.2), no support for older version.