Bug 24064: DUEDGST typoed as DUEGST

Bug 24064: DUEDGST typoed as DUEGST

Bug 20478 introduced a typo where the letter code DUEDGST was changed
to DUEGST. This patch fixes it.

To test:
- Run "grep -r DUEDGST *" on the Koha git repo. Notice that this letter
  code is used in e.g. sample notices.
- Run "grep -r DUEGST *" and notice that this code only occurs twice,
  in misc/cronjobs/advance_notices.pl.
- Aply this patch.
- Run "grep -r DUEGST *" again, and notice there are no more occurences
  of this typo.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>