MdeModulePkg/ConSplitter: Change StdErr color to EFI_LIGHTGRAY

MdeModulePkg/ConSplitter: Change StdErr color to EFI_LIGHTGRAY

ConSplitter is using EFI_LIGHTGRAY foreground color for ConOut
and EFI_MAGENTA for StdErr consoles. This is impacting the DebugLib
output on that same serial console (e.g. DebugLibSerialPort) after
gEfiStandardErrorDeviceGuid is installed on that port. The impact
also extends to Linux serial console output in OVMF because it inherits
the color setting from the firmware.

This is inconsistent and annoying, with MAGENTA being barely legible on
a black background.

Let's change StdErr default color to LIGHTGRAY for consistency and
readability. This results in the same color being used for all consoles
sharing the same serial port (ConOut, StdErr, DebugLib, OS console).
Platforms wishing to distinguish the colors of consoles can do so in
their own Platform BDS initialization.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Ard Biesheuvel <Ard.Biesheuvel@arm.com>
Signed-off-by: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>
(On an RPi 4 platform where this was another annoyance)