docs(admin/external_storage): Specify that Region is a required field

[Christopher-Hayes]

Nextcloud always sends eu-west-1 as the region. For S3 store providers that support the region field, this can cause external storage to not work.

Removes wording that says Region is optional. Clarifies that for providers that use Region, the user must provide it because the Nextcloud default will always be incorrect.

☑️ Resolves

• Fix Clarify that the "region" field is required for pre-existing AmazonS3 buckets #9507

🖼️ Screenshots

Changes the following text since Region is required for some providers, such as Cloudflare R2.

[github-actions]

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