remove untested branches (1.6 series) from Prow

Remove:
- CAPM3 1.6 branch
- BMO 0.5 branch
- Ironic-image 24.0 branch

As they fall of from tested or maintained status.

Remove all job triggers that launch 1.6 related jobs.
Remove milestoneapplier config for 1.6 as well.

Signed-off-by: Tuomo Tanskanen <tuomo.tanskanen@est.tech>

jenkins/README.md

@@ -35,10 +35,6 @@ the commands below. The job result will be posted as a comment.
   version v1beta1 and branch release-1.7 on Ubuntu
 * **/test-centos-integration-release-1-7** run integration tests with CAPM3 API
   version v1beta1 and branch release-1.7 on CentOS
-* **/test-ubuntu-integration-release-1-6** run integration tests with CAPM3 API
-  version v1beta1 and branch release-1.6 on Ubuntu
-* **/test-centos-integration-release-1-6** run integration tests with CAPM3 API
-  version v1beta1 and branch release-1.6 on CentOS
 
 ### E2E tests