[kwin] Remove the legacy OpenGL 1 compositing backend

KWin already has a de facto OpenGL 2 dependency through QML. Combined
with the fact that the OpenGL 1 backend is basically unmaintained and
also unused, it's better to remove it for the new major release.

This change includes:
* Removal of cmake option KWIN_BUILD_OPENGL_1_COMPOSITING
* Removal of KWIN_HAVE_OPENGL_1 compile option and all code
ifdef'ed with it (partially removal of if-else constructs)
* Removal of CompositingType::OpenGL1Compositing (flags are kept
as a core flag should get introduced)
* Driver recommendation for OpenGL1Compositing changed to XRender
(should be evaluated whether the drivers can provide GL2)
* Removal of configuration option "GLLegacy"
* Removal of fooMatrix function in kwinglutils
* Removal of ARBBlurShader
* Removal of legacy code path in GLVertexBuffer
* Removal of GLShaderManager::disable
* if-blocks with ShaderManager::instance()->isValid() removed

REVIEW: 116042