Bug 36151: Update leaflet.js

Bug 36151: Update leaflet.js

Koha currently comes with leaflet.js 1.0.3 which is quite old (2017).
For GeoSearch (Bug 31652) we need a more current leaflet.js, so this
patch updates leaflet.js to the current version 1.9.4

Test plan:

* Go to staff /cgi-bin/koha/admin/usage_statistics.pl where you see a
  map and can set the location of the library by clicking on the map.
* Apply the patch
* Again, go to staff /cgi-bin/koha/admin/usage_statistics.pl
* Do a complete reload of the page
* It should still work:
  * Display the location as a pin
  * Set a new location

There was no code change necessary besides downloading the current
version of leaflet.js and updating the about-page

Sponsored-by: Geosphere Austria
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>