Update Enterprise-tier plans section and billing video on upgrade instructions page

### Contributions

- [x] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/faqs/Accounts/platform-upgrade-instructions?version=2.0&name=Fusion

### What part(s) of the page would you like to see updated?

**Updates needed:**

- Rename the heading to Enterprise-tier plans and update the copy to cover Enterprise and Enterprise+ (consistent with Billing FAQs and other docs).
- Review and update the steps (Billing tab, Contact Sales flow) so they match the current UI.
- Replace or remove the outdated enterprise-upgrade.gif — it no longer reflects the billing page.
- Review the Starter plan screenshots (`trial-team-flow.png`, `trial-team-payments-flow.png`) while updating assets, if those also look stale.

**Page**: [How to upgrade a dbt account](https://docs.getdbt.com/faqs/Accounts/platform-upgrade-instructions)
**File**: `website/docs/faqs/Accounts/platform-upgrade-instructions.md`

### Additional information

_No response_