Update container registry release timing

Why is this change being made?

The due date of the release plan issue has been updated to 10 days before the target GitLab release, to match the new GitLab release schedule.

10 days before the GitLab release has always been the target. This MR explicitly mentions this as opposed to saying "the 12th of the month" -which happened to be 10 days before the old GitLab release dates.

Related to gitlab-org/container-registry!1458 (merged)

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
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  • Team: For changes that affect your direct team, share in your group Slack channel
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