Update "Software update" page re Open Collective

Our existing https://rockstor.com/docs/interface/system/update_channels.html page is in need of a few updates regarding our new Open Collective arrangements, and our discontinued use, as of 1st July 2023, of our now legacy shop.rockstor.com.

Related issues for context:

[x] "Setup legacy shop.rockstor.com landing page": https://github.com/rockstor/rockstor-website/issues/85

Related Pull requests:

Changes in our Web-UI embedded instructions for our transition period from legacy shop to Open Collective plus legacy shop:

[x] "Replace 'SHOP' with 'OPEN COLLECTIVE' in header menu ...": https://github.com/rockstor/rockstor-core/pull/2539

Related forum thread

Appman maintenance and Open Collective intergration.

https://forum.rockstor.com/t/appman-rockstor-com-maintenance-open-collective-integration/8867

N.B. this update may have to be done in stages as we alter available arragements. But primarily we no longer use the shop subdomain, So we need to initially remove links to that in order to avoid frustration with giving folks a run-around.

rockstor / rockstor-doc