wallet: Change default confirm target from 2 to 6

Recent discussion (in IRC meetings, and e.g. #8989) has shown a
preference for the default confirm target for smartfees to be 6 instead
of 2, to avoid overpaying fees for questionable gain.

6 is also a compromise between the GUI's pre-#8989 value of 25 and the
bitcoind -txconfirmtarget default of 2. These were unified in #8989,
but this has made the (overly expensive) default of 2 as GUI default.