Renames delta time to be a more obvious name (#74654)

This tracks the seconds per tick of a subsystem, however note that it is
not completely accurate, as subsystems can be delayed, however it's
useful to have this number as a multiplier or ratio, so that if in
future someone changes the subsystem wait time code correctly adjusts
how fast it applies effects

regexes used

git grep --files-with-matches --name-only 'DT_PROB' | xargs -l sed -i
's/DT_PROB/SPT_PROB/g'
git grep --files-with-matches --name-only 'delta_time' | xargs -l sed -i
's/delta_time/seconds_per_tick/g'

code/modules/modular_computers/file_system/program.dm

@@ -88,7 +88,7 @@
 	return TRUE
 
 // Called by Process() on device that runs us, once every tick.
-/datum/computer_file/program/proc/process_tick(delta_time)
+/datum/computer_file/program/proc/process_tick(seconds_per_tick)
 	return TRUE
 
 /**