Get rid of the hardcoded versions in the CI pipeline

[onobc]

This is improvement ticket.

In ci.yml we have to list the docker images to push to Docker registry. The image includes a hardcoded version number.

It would be an improvement to not require this version number that is controlled via the pom.xml. This introduces toil when the version number needs to be changed.

It would be even nicer if the list of docker-images did not need to be provided and instead one of the GH actions output the proper list of image names to push.

  user-clicks-per-region-12x:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: ./.github/actions/build-sample-app
        with:
          app-dir: 'multi-io-samples/kafka-streams/user-clicks-per-region-1.2.x'
          mvn-build-commands: '-B clean install spring-boot:build-image -pl http-click-ingest,http-region-ingest,http-clicks-and-region-ingest,user-clicks-per-region-processor,log-user-clicks-per-region'
          ....
          docker-images: >-
            springcloudstream/multi-io-sample-http-click-ingest:2.0.0-SNAPSHOT
            springcloudstream/multi-io-sample-http-region-ingest:2.0.0-SNAPSHOT
            springcloudstream/multi-io-sample-http-clicks-and-region-ingest:2.0.0-SNAPSHOT
            springcloudstream/multi-io-sample-user-clicks-per-region-processor:2.0.0-SNAPSHOT
            springcloudstream/multi-io-sample-log-user-clicks-per-region:2.0.0-SNAPSHOT