Drop MinIO migration documentation

What does this MR do?

Drop MinIO migration documentation

• Drop docs to migrate to MinIO and link the stateful service migration documentation instead.
• Clarify status of bundled charts in package to helm migration.

gitlab-org/technical-writing/docs-gitlab-com!1736 (comment 3000125699)

Related issues

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.