Object storage docs are not relevant to GitLab.com and is in Core

What does this MR do?

Remove reference to Silver from the generated docs because this is an instance admin feature, and update to reflect this feature was moved to Core in 10.7 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17358

Related issues

Related to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3867

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards