Bug 12224: (follow-up) Format today with KohaDates

Bug 12224: (follow-up) Format today with KohaDates

Updates the slip templates to use [% today | $KohaDates %].
Adding an explicit USE statement is not necessary in this case.

To test:
- When testing the feature according to the test plan, make sure
  that the date for today shows up formatted to the DateFormat
  system preference.

Note: As we are printing today's checkins, I have intentionally left
oft the time. But it could be easily added if a library wanted it.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>