Update SaaS New Subscription Mailer to be more generic

## Problem

The current sales-assisted "new subscription" mailer is sent to all customers who purchase a SaaS subscription (new, renewal or updated) that isn't linked to a namespace. The wording is currently slightly different by including or excluding the word "new" in the email subject and body depending on a new subscription purchase or an update purchase to the subscription.

## Proposal

Make the SaaS new subscription mailer more generic by:

* Updating the first sentence from `Thanks for purchasing a GitLab subscription.` to `Thank you for your GitLab purchase!`
* Updating the second sentence from `To set up your new subscription` to `To set up your subscription`

### Details

<details><summary>Current email screenshot</summary>
![image](/uploads/a9a9658443afb73ac17acb2dc87215a5/image.png)

</details>


<details><summary>Current email text</summary>

``` 
	
Hi Courtney👋
Thanks for purchasing a GitLab subscription. To set up your new subscription, you must link it to a group. Complete the following instructions.

1. Create your GitLab account
   If you are already registered, sign in.

   Register

2. Select the group for your subscription
Create a new group or for an existing group, make sure you have the Owner role.

3. Sign in to your Customers Portal account
    * If you purchased from our Sales team, we've sent you an email with instructions to set a password and access your Customers Portal account.
    * If you purchased from an authorized reseller, you won't yet have access to your Customers Portal account. Contact our support team to complete the remaining steps.

4. Link your GitLab account and subscription
    * Link your GitLab user account to your Customers Portal account.
    * Link the subscription to your namespace.
  
5. Confirm your subscription is linked
To confirm your subscription is linked to a group, view your GitLab SaaS subscription.

If you require assistance, contact GitLab support.


Thanks,

The GitLab Team	
```

</details>






### Text Proposal

``` 
	
Hi Courtney👋
Thank you for your GitLab purchase! To set up your subscription, you must link it to a group. Complete the following instructions.

1. Create your GitLab account
   If you are already registered, sign in.

   Register

2. Select the group for your subscription
Create a new group or for an existing group, make sure you have the Owner role.

3. Sign in to your Customers Portal account
    * If you purchased from our Sales team, we've sent you an email with instructions to set a password and access your Customers Portal account.
    * If you purchased from an authorized reseller, you won't yet have access to your Customers Portal account. Contact our support team to complete the remaining steps.

4. Link your GitLab account and subscription
    * Link your GitLab user account to your Customers Portal account.
    * Link the subscription to your namespace.
  
5. Confirm your subscription is linked
To confirm your subscription is linked to a group, view your GitLab SaaS subscription.

If you require assistance, contact GitLab support.


Thanks,

The GitLab Team	
```

</details>


### Result
Mailer is relevant to all customers buying a GitLab.com subscription without an associated namespace.

Story points: 1.0