docs/relnotes: add support for GL_ARB_gl_spirv, GL_ARB_spirv_extensions and OpenGL...

docs/relnotes: add support for GL_ARB_gl_spirv, GL_ARB_spirv_extensions and OpenGL 4.6 on i965 and iris

After 41549a18e6c ("i965: Enable OpenGL 4.6 for Gen8+"), i965
implements GL_ARB_gl_spirv, GL_ARB_spirv_extensions and OpenGL 4.6.

After 15e439071d8 ("iris: Enable ARB_gl_spirv and ARB_spirv_extensions"),
iris implements GL_ARB_gl_spirv, GL_ARB_spirv_extensions and OpenGL
4.6.

v2:
  - Explicit the support is for i965 and iris.

v3:
  - Add also GL_ARB_spirv_extensions to the release notes (Alejandro).

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>