Bug 25149: Fix Zebra language code for Greek

Bug 25149: Fix Zebra language code for Greek

According to the list of two-letter language codes found in ISO 639-1
(https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes), the correct
Zebra language option for Greek should be 'el', not 'gr'.

This patch fixes that.

Test plan:
0) Apply the patch.
1) Confirm that 'gr' has been changed to 'el' in all relevant files.
2) Create a new Koha instance using `koha-create --zebralang el testinst1`.
3) Edit /etc/koha/koha-sites.conf and set ZEBRA_LANGUAGE to 'el'. Then,
   create another Koha instance with `koha-create testinst2`.
4) Confirm that both instances created in steps 2) and 3) have picked up
   the Zebra CHR file from the /etc/koha/zebradb/lang_defs/el/ directory.
   This is done by inspecting the value of 'profilePath' in these files:
   /etc/koha/sites/testinst{1,2}/zebra-authorities-dom.cfg
   /etc/koha/sites/testinst{1,2}/zebra-biblios-dom.cfg

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>