Create SaaS Subscription Update specific mailer

## Problem
Today, SaaS customers do not receive an email when they update their subscription (i.e. add seats). This is not a great customer experience, because they have no confirmation that the purchase went through.


## Proposal
Similar to what was done in https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4543 for Cloud Licensing SM customers, setup a new mailer for subscription updates (e.g. new seats, upgrade plan, etc.) for SaaS customers that:
1. Provides a confirmation to customers that their additional purchase was successful, in a way that makes it clear this was an update/add-on to an existing subscription
1. Gives the customer an audit trail to follow in email

https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5057+ will implement a similar email for renewals.

Add the new mailer to the [mailers doc](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/doc/mailers/index.md).


### Proposed Template

NOTE: this will apply for additional seats, CI Minutes, Storage or an upgrade, so it needs to be somewhat vague.

<details><summary>Current email screenshot</summary>
![image](/uploads/a9a9658443afb73ac17acb2dc87215a5/image.png)

</details>

```

Hi Courtney👋
Thanks for your GitLab purchase! 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. Amendment Type is `NewProduct`, `UpdateProduct` or `UpdateSubscription`.
1. Namespace info is present.
   1. If the namespace info is missing the unlinked SaaS purchase email should be used instead.
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), same as used for https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5057+) until the voucher email is implemented.

**Note:** https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/6086+ should be completed directly after this one.

### Result
1. Customer receives an email confirmation when a SaaS product update is completed that clearly outlines an add-on/upgrade occurred.

### Availability & Testing

<!-- This section needs to be retained and filled in during the workflow planning breakdown phase of this feature proposal, if not earlier.

What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?

Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change

See the test engineering planning process and reach out to your counterpart Software Engineer in Test for assistance: https://about.gitlab.com/handbook/engineering/quality/test-engineering/#test-planning -->

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 update? During creation?

Story points: 2.0