Update shader baking versions for compute

Compute shaders and storage buffers require OpenGL 4.3+,
3.2+ for GLES to support image atomics - pre 3.2 requires
shader_image_atomic extension.

Changes the non-persistent use versions to use the current format
version, plus a floor version alongside it so that a silent translation
error does not leave the GLSL outputs empty.

Moves that version selection into glslTargetsForContext() so that it can
be exercised without a live context, and adds a test for it and for the
target list of the persistent baker.

Task-number: QTBUG-146226
Change-Id: Ie5d48b5ff6b64de374b0b1828ccc3588c40ea1f6
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>