Rename "Errungenschaft" to "Vermerk"

Description

The name "Errungenschaft" isn't nice and should be replaced by "Vermerk"

Goal

Improved understandability

Evidence

Long discussion in the product team with dozens of name suggestions lead to "Vermerk".

Solution

Rename only in translation files. In the code, we can keep using "achievement".

Links / references

Name discussion: https://beta.foodsharing.de/region?bid=2296&sub=forum&tid=268168