Update the renewal banner copy to say "Purchase a new subscription"

<!-- The first four sections: "Problem to solve", "Intended users", "User experience goal", and "Proposal", are strongly recommended, while the rest of the sections can be filled out during the problem validation or breakdown phase. However, keep in mind that providing complete and relevant information early helps our product team validate the problem and start working on a solution. -->

### Problem to solve

<!-- What problem do we solve? Try to define the who/what/why of the opportunity as a user story. For example, "As a (who), I want (what), so I can (why/value)." -->

GitLab.com allow 14 days of grace period before a namespace is downgraded to free. The banner state that the user has 14 days to **renew** the subscription with a `Renew subscription` button. When the customer goes to [customers portal](https://customers.gitlab.com/customers/sign_in) to renew the subscription, the renew button doesn't exist because the Bronze/Silver/Gold subscription has already expired.

### Intended users

<!-- Who will use this feature? If known, include any of the following: types of users (e.g. Developer), personas, or specific company roles (e.g. Release Manager). It's okay to write "Unknown" and fill this field in later.

Personas are described at https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/

* [Parker (Product Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#parker-product-manager)
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
* [Presley (Product Designer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#presley-product-designer)
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
* [Rachel (Release Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#rachel-release-manager) 
* [Alex (Security Operations Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#alex-security-operations-engineer)
* [Simone (Software Engineer in Test)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#simone-software-engineer-in-test)
* [Allison (Application Ops)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#allison-application-ops) 
* [Priyanka (Platform Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#priyanka-platform-engineer)
-->

Unknown

### User experience goal

<!-- What is the single user experience workflow this problem addresses? 
For example, "The user should be able to use the UI/API/.gitlab-ci.yml with GitLab to <perform a specific task>"
https://about.gitlab.com/handbook/engineering/ux/ux-research-training/user-story-mapping/ --> 


### Proposal

<!-- How are we going to solve the problem? Try to include the user journey! https://about.gitlab.com/handbook/journeys/#user-journey -->

~~Allow the renew button to persist during the (grace period) so that customer can renew their subscription without confusion.~~

Update the button copy to speak to purchasing a new subscription vs. renewing

Story points: 2.0