Review the upgrade guides `R5->R6` and `Migration to KaaS v2` and update if necessary

[matofeder]

As a KaaS administrator, I want smooth KaaS upgrades based on the docs guides.

Consider the latest CAPO API changes and whether they could affect our upgrade guides in docs.

[Nils98Ar]

Feedback:

• The first option here (with the fixed versions) should be used everywhere
• This part seems (partly) redundant with this part
• Add a hint here and here how to do the update of the files manually, especially cluster-template.yaml. We diffed our customized R5-cluster-template.yaml with the original R5-cluster-template.yaml and applied the diff to the new R6-cluster-template.yaml.
• The maintained/7.* branches mentioned in the upgrade guide are not available.
• Maybe add support for CAPO API v1beta1? Or in the next release...
• Here the OPENSTACK_CLUSTER_GEN needs to be incremented as well. Otherwise the create_cluster.sh had problems with immutable specs values in our case.
• The check if the cluster-template.yaml has already been updated could be improved. In our case the grep found occurrences of provider-id already before the update (probably we used the main branch in an older state) but the rest was not there. Maybe it would be good to replace the or with and and in the grep?