Update release notes for 3.12.15 release

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Translations updates for Koha 3.12.15 release

Added new language Khmer.
Merge remote branch 'koha-translate/3.12.15-translate' into HEAD
Increment version for 3.12.15 release

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>