Do not store `application_settings.static_objects_external_storage_auth_token` tokens in plaintext

Problem

With gitlab-foss!31025 (merged), we introduced new token fields (application_settings.static_objects_external_storage_auth_token) without digesting or encrypting the value.

Proposal

We shouldn't store any in plaintext and we should also migrate existing ones.

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A follow-up issue for users.static_object_token #348163 (closed)