Update EGL_GL_COLORSPACE_BT2020_HLG_EXT to the correct value

ag/22764137 added support for HLG formats, but the spec change in
the Khronos gitlab hadn't landed yet. The enum I used had already been
used by an unpublished NVIDIA EXT, so I needed to choose a different
one. This change uses the new one which is the final correct enum found here:

https://github.com/KhronosGroup/EGL-Registry/commit/526f9a6471106090e44b8d6a123f78886d5d88fa

Bug: 277210442
Test: Manually ran new test in CL https://gerrit.khronos.org/c/vk-gl-cts/+/11608
Change-Id: I5c163b7ef2c3e6fa6f90ac8de6f75a7b7e67cf8c