[ATLASRECTS-6630] Update TriggerConfigFlags.py to change Trigger.doNavigationSlimming default to True

47242! unifies doNavigationSlimming to work for R2 and R3 data.

Currently we still write the minimally slimmed R2 navigation format to the AOD.

This MR switches the default to true, enabling R3 navigation slimming too.

NOTE: This MR on its own will not change any behaviour, this is because there is a priority-list, and the top-priority collection, HLTNav_Summary_OnlineSlimmed is still stored in ESD and AOD. In order for a job to read one of the new HLTNav_Summary_ESDSlimmed or HLTNav_Summary_AODSlimmed, the job's TDT will need to be explicitly configured as such.

NOTE: The slimming only manipulates the navigation graph itself, not any HLT_xxx online collections.

cc @fwinkl @cleonido @jburr

Fixes one aspect of [ATLASRECTS-6630]