On macOS 11+ OpenGL is implemented on top of Metal internally.
This CL changes ANGLE's Metal backend's renderer string to better
differentiate it from the above OpenGL renderer.

Bug: angleproject:5841
Change-Id: I0d5466594e385cb663d537db034c82b006b6e907
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2820179
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>