docs: Remove no-longer-accurate text about the xlib driver
Mesa hasn't supported color-index rendering in a long time, and gallium's xlib target doesn't respect MESA_GAMMA.
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Equinix is shutting down its operations with us on April 30, 2025. They have graciously supported us for almost 5 years, but all good things come to an end.
Given the time frame, it's going to be hard to make a smooth transition of the cluster to somewhere else (TBD). Please expect in the next months some hiccups in the service and probably at least a full week of downtime to transfer gitlab to a different place.
All help is appreciated.
Mesa hasn't supported color-index rendering in a long time, and gallium's xlib target doesn't respect MESA_GAMMA.