x86/uv: Update HPE Superdome Flex Maintainers

Mike Travis has retired. His expertise will be sorely missed.
Remove Mike's entry under SGI XP/XPC/XPNET DRIVER.
Replace Mike's entry under UV HPE SUPERDOME FLEX.

Signed-off-by: Justin Ernst <justin.ernst@hpe.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Joel Granados <j.granados@samsung.com>
Link: https://lore.kernel.org/all/20230801155756.22308-1-justin.ernst%40hpe.com