Bump version to v0.12.0

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Merge pull request #6230 from tiborvass/bump_v0.12.0

Bump version to v0.12.0
Change version to 0.12.0-dev

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Merge pull request #6238 from tiborvass/merge_release_v0.12.0

Merge release v0.12.0
Mention bugfixes in 0.12 changelog