Add section about testing uploads to package team

Why is this change being made?

Recently, a production incident was triggered with an underlying cause that would not have been prevented unless testing against a specific cloud storage provider for object storage.

When using GDK for development, the default environment uses local storage, and the common way to test against object storage is to use MinIO. Given that the package stage deals directly with file uploads in many of our major features, we should be more conscious of trying to better replicate the production environment. In the case of the mentioned incident, the bug was only triggered when using GCP for object storage (which is what GitLab.com uses), but the other providers all worked properly.

This MR adds a section to our handbook page saying that we should take this into account when testing and prefer using a cloud service by default over MinIO when testing object storage capability, perhaps GCP since that would best replicate the GitLab.com environment.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)