Deprecate Unified License mailers & remove SS aware check

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### Background
We currently have 4 mailers that can be sent to customers when a Self Managed subscription is purchased - a legacy specific mailer, an offline license specific mailer, a cloud license specific mailer, and a unified mailer.

The unified mailer was initially established in the early days of Cloud Licensing and contained _both_ the legacy license and a cloud license activation code. This was sent to customers who had `TurnOnCloudLicensing` set to `null`. As part of SCL, we removed the legacy license from this mailer as we began to treat `TurnOnCloudLicensing` = `null` customers the same as `TurnOnCloudLicensing` = `Yes`. Therefore it is now a duplicate of the activation code mailer.

In addition, the check for `super_sonics_aware` is no longer necessary. Since we are over 1 year past the launch of Super Sonics (August 2021), all new or renewal subscriptions will now be super sonics aware. This can be removed as unnecessary logic.

Related part of code:
![image](/uploads/c8fad86be7698188a6258e238c1f69ca/image.png)

### Proposal
Deprecate the unified license mailer and simplify the existing mailer logic to send only the legacy, offline or activation code mailer as is appropriate based on the license type the customer will receive, with no dependency on `super_sonics_aware`.

Rough suggestion:
* If legacy, send legacy mailer
* If offline, send offline mailer
* Else, must be online, so send online mailer (reordered as I think online mailer should be the default)

In addition, we should deprecate the related [unified voucher email](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/spec/mailers/unified_license_voucher_mailer_spec.rb), as this will no longer be needed.

### Result
Simplification in our customer mailers.

### Availability & Testing

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* Integration test changes
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This refactor change will likely result in a lot of changed files and tests. On hand-hand, this should be a simple consolidation however on the other hand, we risk breaking something. 

We should be able to move all existing tests from our other mailers into one mailer as proposed, but it's possible more will have to be done to ensure everything works properly.  

From a ~Quality perspective, let's make sure these MRs are reviewed from the ~test perspective and that we don't suffer test coverage loss due to this change.

Story points: 2.0