Check syntax of release notes file in CI

[ZanSara]

Right now we check for the presence of this file, but not for its syntax.

This leads to surprise failures in the generation of the release notes at release time, like this: https://github.com/deepset-ai/haystack/actions/runs/8002246058/job/21855066081

Let's check for the syntax in CI to prevent these issues.