Create SaaS renewal specific mailer

## Problem
Today, SaaS customers receive the same new purchase email no matter if they are purchasing a new subscription, renewing an existing, or simply adding a few seats. This is not a great customer experience, especially given how "new purchase" oriented the current email is.


## Proposal
Similar to what was done in https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4542 for Cloud Licensing SM customers, setup a new mailer for renewing SaaS customers that:
1. Provides a confirmation to customers that their subscription renewal was successful, in a way that makes it clear this was a renewal and not a new subscription purchase
1. Gives the customer an audit trail to follow in email

https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5058+ will implement a similar email for subscription updates.

Add the new mailer to the [mailers doc](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/doc/mailers/index.md).

### Proposed Template

<details><summary>Current email screenshot</summary>
![image](/uploads/a9a9658443afb73ac17acb2dc87215a5/image.png)

</details>

```

Hi Courtney👋
Thanks for renewing your GitLab SaaS subscription! We appreciate your loyalty as a GitLab customer!

Your purchase will be automatically applied to the same namespace on the start date listed below. Here are the details of your subscription:

1. `product name`
2. `term start date`
2. `term end date`
3. `quantity`

If you require any assistance, contact GitLab support.


Thanks,

The GitLab Team

```

Note: keep the existing link to GitLab Support (https://about.gitlab.com/support/#contact-support) in the last line.

### Technical Requirements
1. Send these mailers for all subscription updates when:
   1. Purchase is of a SaaS subscription
1. Order Type = `Renewal`
1. Namespace info is present.
   1. If the namespace info is missing the new subscription mail should be used instead.
   1. Remove the reference to "new subscription" in the email subject and body.
1. Update logic to send the correct mailer based on subscription type (Renewal, Amendment, New Subscription)
1. Ensure the newer version of the template design is used, consistent with the current New Subscription SaaS purchase email
1. Put the new mailer behind a feature flag ([`new_saas_mailers`](https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags/209/edit)) until the voucher email is implemented.

**Note:** https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/6085+ should be completed directly after this one.

### Result
1. Customer and revenue receive an email confirmation when a SaaS renewal is completed that clearly outlines a renewal occurred.


### Availability & Testing

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Adding a mailer, should be accompanied by several tests.

- Mailer Specs
  - Mailer Subject
  - Mailer To/From (multiple To's? If so, which is first?)
  - Mailer Body?
- When is this Mailer activated?  Only during renewal?

Story points: 3.0