Include major version decision in migration guide

[dagood]

• Bring a part of Revise documentation, include digest pinning go-images#659 over to this document, because it really applies to any installation method.

Try to be a lot more direct about our recommendations and the underlying reasoning.

Try using "current" and "previous" terminology with translation for other (apparently) less common terms.

Break out a dependency pinning section. For now, focuses on the existing topic of pinning to a major version, not immutable dependency pinning to specific builds with auto-updates through Dependabot etc.

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[Copilot]

Pull request overview

Updates the migration guide to be more explicit about how to choose a supported Microsoft build of Go “major version” (current vs previous) and how to manage version pinning during and after migration.

Changes:

• Adds a “What version should I use?” section defining current/previous and recommending selection criteria.
• Adds a “Dependency pinning” section describing why and how to pin to a major version range (e.g., 1.26.*).
• Links the quick-start steps to the new version-selection guidance.