Add an SPF Record for test.cloud.mattermost.com (TEST) domain

Description

In order to reduce bounce rates, AWS recommends a few actions to improve email realiability. One of them is to implement SPF aka Sender Policy Framework. Read more about it here: https://docs.aws.amazon.com/ses/latest/DeveloperGuide/send-email-authentication-spf.html

Snippet from the site above:
```
Adding an SPF Record
To publish an SPF record, you have to add a new TXT record to the DNS configuration for your domain. The procedures for updating DNS records vary depending on which DNS or web hosting provider you use.

The following table includes links to the documentation for several common providers. This list isn't exhaustive, and inclusion in this list isn't an endorsement or recommendation of any company's products or services. If your provider isn't listed in the table, you can probably still publish an SPF record.
...
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QA Test Steps

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Mana

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Assignee

Unassigned

QA Assignee

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Reporter

Ali Farooq

Epic Link

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Fix versions

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Mattermost Team

Cloud SRE

Sprint

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Labels

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QA Testing Areas

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GitHub Issue

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Components

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Configure