docs: Update Migration Guide version mapping table for current GitOps releases

[sudhishmk]

Is your feature request related to a problem?

The "Which GitOps Operator version should I migrate to?" table in docs/Migration_Guide.md only lists GitOps v1.3–v1.5 and Argo CD v2.1–v2.3. Current releases are v1.20.x with Argo CD v3.3.x.

Describe the solution you'd like

Extend the version mapping table with current GitOps Operator, Argo CD Community Operator, and default Argo CD versions (e.g. v1.20/v1.19/v1.18 and v1.16/v1.15), with a link to RH release notes for exact patch versions.

Additional context

• Migration guide: https://github.com/redhat-developer/gitops-operator/blob/master/docs/Migration_Guide.md#which-gitops-operator-version-should-i-migrate-to-
• RH 1.20 release notes: https://docs.redhat.com/en/documentation/red_hat_openshift_gitops/1.20/html/release_notes/gitops-release-notes

[sudhishmk]

I would like to work on this. /assign @sudhishmk