ply-terminal-emulator: Fix incorrect SGR attribute handling

ply-terminal-emulator: Fix incorrect SGR attribute handling

According to ECMA-48 [1], Select Graphic Rendition (SGR) attribute 21
is for "doubly underlined", not "disabling bold" in current plymouth
implementation.

Additionally, SGR 22 is for "normal colour or normal intensity (neither
bold nor faint)", both bold and dim should be disabled with it.

[1] 8.3.117, https://www.ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>