Update release note fetcher to get Operate releases from monorepo

[pedesen]

Description

The Operate 8.5.0 release (and future releases) will be published in the zeebe monorepo instead of the operate repo (which causes the release-notes-fetcher workflow to fail):

3:16PM ERR An error has occurred error="GET https://api.github.com/repos/camunda/operate/releases/tags/8.5.0: 404 Not Found []

TODO

• Update release-notes-fetcher/fetch.go
• Update release-notes-fetcher/release.sh
• Check if other places needs to be updated

The Operate release notes are now located in different repos depending on the release version:

8.5.x and newer: repo: camunda/zeebe, tag: operate-8.x.x (please note the prefix)
8.4.x and older: repo camunda/operate , tag: 8.x.x

Here is the 8.5.0 release as example: https://github.com/camunda/zeebe/releases/tag/operate-8.5.0