evolution: rename trouble(s) to instability
Rename trouble(s) to instability in all external user-facing output. Only
update user-facing output for the moment, variables names, templates keyword
and potentially configuration would be done in later series.
The renaming is done according to
https://www.mercurial-scm.org/wiki/CEDVocabulary.
Differential Revision: https://phab.mercurial-scm.org/D213