Bump minimum GL/GLES versions
There is now only a GLSL backend for cogl, so we can simplify a lot of this.
Technically one could support GLSL on OpenGL 1.x if you implemented GL_ARB_shading_language_100 and friends. Practically speaking there are no such drivers in the world, and even if they did exist they'd be from approximately 2004-2007 so it's unlikely they'd work very well on a modern system anyway.