st/mesa: only require ESSL 3.1 for geometry shaders
According to the OES_geometry_shader spec, section Dependencies:
"OpenGL ES 3.1 and OpenGL ES Shading Language 3.10 are required."
According to the OES_geometry_shader spec, section Dependencies:
"OpenGL ES 3.1 and OpenGL ES Shading Language 3.10 are required."