Update docs for GoTool@0, update quick start

[dagood]

• Remove another 1.24 complication.
• Clarify the pinning recommendation.
• Document the results of Make microsoft/go releases acquirable via GoTool@0 Azure Pipelines task #483

[Copilot]

Pull request overview

Updates documentation to reflect that Azure Pipelines’ built-in GoTool@0 task can install the Microsoft build of Go, and clarifies guidance around version pinning in CI/build environments.

Changes:

• Removes the quick-start callout about manually enabling systemcrypto for pre-1.25 toolchains.
• Reworks the pinning recommendation into a TIP block with clearer tradeoffs.
• Documents how to use GoTool@0 with goDownloadUrl to install the Microsoft build of Go, and links to that guidance from the README.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
eng/doc/MigrationGuide.md	Updates quick-start/pinning guidance and adds GoTool@0 instructions for Microsoft Go.
README.md	Adds an “Azure Pipelines GoTool@0 task” section pointing readers to the Migration Guide, and moves “Binary archive” down.

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