Maintainers.txt: update UEFI payload information
    
    Remove CorebootModulePkg and CorebootPayloadPkg, and add
    UefiPayloadPkg to reflect recently change in UEFI payload.
    wiki link for UefiPayloadPkg would be available soon.
    
    Signed-off-by: Guo Dong <guo.dong@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>
    Reviewed-by: Maurice Ma <maurice.ma@intel.com>