Fixes#81758
DisplayServerWeb::process_joypads handles buttons 6 and 7 of the
HTML5 Standard Gamepad as a special case by doing:
`input->joy_axis(idx, (JoyAxis)b, s_btns[b]);`
This doesn't work because there is no JoyAxis 6 or 7 in the enum
To fix this we use JoyAxis::TRIGGER_LEFT and TRIGGER_RIGHT for button 6
and 7
However since we are now lying to input->joy_axis we also need to lie in
the mappings for the standard gamepad in godotcontrollersdb.txt,
otherwise input->joy_axis will try to find a mapping to axis 4(LT) and
axis 5(RT) that's not defined.
Therefore we set lefttrigger to +a4 and righttrigger to +a5 in the
mapping, to match what we are actually sending.
A cleaner, and more involved fix to this would be modifying
input->joy_button so that it can handle analog buttons and map them to
axes preserving their value instead of converting to boolean
9dd372f316