Stop linking to `6.1.22-SNAPSHOT` reference docs

[sbrannen]

Overview

The version selector for the reference docs still lists 6.1.22-SNAPSHOT even though we no longer build the 6.1.x branch.

Deliverables

We should update the branches configuration in antora-playbook.yml to ensure that we are only publishing snapshot documentation for 6.2.x and main (which currently correlates to 7.0.x), and we should revise the configuration in such a way that future branches such as 7.0.x, 7.1.x, etc. are also supported.

• Update the branches configuration in antora-playbook.yml on main branch.
• Update the branches configuration in antora-playbook.yml on docs-build branch.