docs: update Migration Guide version mapping for current releases

[sudhishmk]

Extend the community-operator-to-GitOps migration table through v1.20 and add a pointer to RH release notes for exact patch versions. Fixes #1279

What type of PR is this?
/kind documentation

What does this PR do / why we need it:
Updates the version mapping table in docs/Migration_Guide.md so users migrating from Argo CD Community Operator can choose a current GitOps Operator release (through v1.20.x).

Have you updated the necessary documentation?

• Documentation update is required by this PR.
• Documentation has been updated.

Which issue(s) this PR fixes:
Fixes #1279

Test acceptance criteria:

• Unit Test
• E2E Test

How to test changes / Special notes to the reviewer:
Docs-only change. Please verify version rows against RH OpenShift GitOps 1.20/1.16/1.15 release notes.

[openshift-ci]

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Hi @sudhishmk. Thanks for your PR.

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📒 Files selected for processing (1)

• docs/Migration_Guide.md

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• argoproj-labs/argocd-operator (manual)

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📝 Summary

Summary by CodeRabbit

• Documentation
  • Updated the migration guide’s version compatibility table with current GitOps and Argo CD Operator mappings.
  • Added support information for newer Argo CD versions.
  • Added a reference to Red Hat OpenShift GitOps release notes for exact component versions.

Walkthrough

The migration guide updates its version compatibility table with GitOps Operator releases from v1.15.z through v1.20.z, corresponding Argo CD Operator versions, default Argo CD versions, and a link to Red Hat OpenShift GitOps release notes.

Changes

Migration guide version mapping

Layer / File(s)	Summary
Update version compatibility mappings docs/Migration_Guide.md	The table replaces older release rows with current GitOps Operator, Argo CD Operator, and default Argo CD version mappings. A release-notes link provides exact component versions.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to 4b9e6

The migration guide now documents newer GitOps Operator and Argo CD version mappings and directs readers to release notes for patch-level details. No current merge-readiness risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change extends the Migration Guide table with the requested current GitOps Operator, Community Operator, and Argo CD versions, and adds a Red Hat release-notes link for exact patch versions [#1279…
Out of Scope Changes check	✅ Passed	The changes are limited to the requested Migration Guide version table and related release-notes reference.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Title check	✅ Passed	The title clearly identifies the documentation change and the updated version mapping.
Description check	✅ Passed	The description accurately explains the migration table update, release-notes link, scope, and testing context.

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