Put v2 upgrade instructions at top of v3 migrate guide

[1000TurquoisePogs]

This PR makes the following changes:

• Moves the "## Special Considerations for Older Versions" to the top as "## Upgrading to the latest version of Zowe v2 (v2.18)"

The reason is users should see this first, not last, to ensure they are on the right version to follow the rest of the guide

• Added additional old version instructions based on my memory of what changed

• Moved the keyrings change out of "prerequisites" and into "configuration changes", because it is a configuration item.

[github-actions[bot]]

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