Update version to 0.12.0, clean up unused build pipelines.

[gigerdo]

Some post-release updates and cleanup of unused files

• update version to 0.12.0 in makefile, examples etc.
• Also removes outdated CI jobs (Jenkins jobs and broken Github action)
  • Technically we could also fix it, but I don't think there is really much value in automatically creating a branch. We can always create it manually if we would ever need it (I don't remember that we ever used it)

[dimuon]

I guess, version branch may be useful if we decide to release a fix for a specific version but, indeed, so far we didn't do it.

[gigerdo]

I guess, version branch may be useful if we decide to release a fix for a specific version but, indeed, so far we didn't do it.

If we need it, it would be simple to manually create a branch at that point.