fix nostr for "br" [BZ #21706]

"Ket" is the the most used negative answer, as it's the negative answer
to a positively phrased question
It's used as it or with the verb ("Ne ran ket", ...)
As such, "Ket" is used in most translations.
"Nann" is less used as it's the negative answer to a negatively phrased
question

See https://en.wikipedia.org/wiki/Yes_and_no for explanations about
languages with 3 or 4 form systems.

We still keep "Nn" for short answers as:
- new learners are used to "Non" in french
- and they often misuses "Nann"
- for compatibility with english

	[BZ #21706]
	* localedata/locales/br_FR (LC_MESSAGES): Fix nostr.