Update product subscription cancellation flow in workspace billing docs

[adambdnr]

Summary

• Updates the Canceling a product subscription section (formerly "Canceling your subscription") to reflect that each product (Bitrise CI, Build Cache, Release Management, etc.) now has its own independently cancellable subscription, rather than one workspace-wide subscription.
• Documents that add-on subscriptions can also be canceled on their own without affecting the parent product's subscription.
• Replaces the old dialog-based cancellation steps with the new Plan & Billing → product card → Manage → drawer → Cancel subscription flow.
• Removes the now-outdated teams-sub.png screenshot reference (the dialog it depicted no longer exists).

Test plan

• Tech writer review of wording/style against the Bitrise Style Guide
• Confirm the new flow description matches the current product UI
• Add/replace screenshot for the product drawer's Cancel subscription step, if desired

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[github-actions]

Preview: https://fix-product-subscription-can.bitrise-docs.pages.dev