Updated PAT and deploy token use with ext auth deprecation notice

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In !108625 (diffs) we issued a deprecation notice around the use of PAT and deploy tokens with external authorization enabled. A similar notice was added by groupcontainer registry but it inadvertantly suggested that External Authorization itself will be enabled by default, which is not the case. Correcting that notice in this MR based on a user highlighting the mistake https://gitlab.com/gitlab-com/account-management/emea/easdale/collaboration/-/issues/131+

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