Further changes for Opencast container images

[mtneug]

Hi all,

Following https://github.com/orgs/opencast/discussions/6478, there are some further changes for the container images:

• I'm moving away from the next image tag. Instead, you can now use {version}-dev for up-to-date builds of Opencast release branches. The dev image will always be built from the develop branch. The next tag will no longer be updated.
• I changed the branching model slightly. There are now version branches named {version}.x. The release branches {version}.{minor_version} will be derived from those branches. The main branch is equal to the latest version branch, i.e. currently 18.x. In the past, the main branch was equal to the latest release branch (i.e., 17.1 before this change). This now better reflects the Opencast branching model.
• New container tags are now also announced in https://matrix.to/#/#opencast-announcements:matrix.org

– Matthias