Fix version upgrade string for RTL languages

Merged username-removed-56399 requested to merge relan/fdroidclient:upgrade-arrow-rtl into master

In right-to-left languages forward is the direction to the left, so leftwards arrow should be used to indicate upgrade:

upgrade-arrow-rtl

For left-to-right languages it remains the same:

upgrade-arrow-ltr

Closes #609 (closed).