Migrate read-only objstorage from moma to dynamic infra

• Add swh/objstorage=true labels to rancher node metal
• swh/infra/ci-cd/swh-charts!272 (merged): Deploy new objstorage multiplexer read-only (in dynamic infra)
• Deal with http authentication (dealt with varnish so noop)
• swh/infra/puppet/puppet-swh-site!676 (merged): Expose new instance through moma
• Doc update (noop since we use the same fqdn) [1]
• Checks
• swh/infra/puppet/puppet-swh-site!678 (merged): Decommission gunicorn-swh-objstorage from moma
• Checks

[1] https://docs.softwareheritage.org/sysadm/network-architecture/service-urls.html

Refs. #5110 (closed)