Set up release-4.11 branch with fast-forwarding in assisted repos

On April 11th, in a meeting with the ecosystem team, it was agreed that in order to preserve the cycle development length for RHACM, development in assisted repos for OCP 4.11 and RHACM-2.6 would evolve in:

"Separate branches option tested with new configuration. Mitigate risks with this approach. Regroup if this doesn’t work as expected."

The new configuration refers to the following repos:

• https://github.com/openshift/assisted-service
• https://github.com/openshift/assisted-installer
• https://github.com/openshift/assisted-installer-agent
• https://github.com/openshift/assisted-image-service

And the content being:

• Main prow fast forwarding configuration making release-4.11 fast forward to tracking master until OCP 4.11 code freeze
• Variant prow fast forward configuration making release-ocm-2.6 fast forward tracking master until RHACM-2.6 feature freeze

Acceptance criteria:

• Both release branches created
• Fast forwarding working on both branches
• Plan for master branch management and backport policy