Move "Update the plugin's pubspec.yaml" outside of tab

[jmagman]

Description of what this PR is changing or adding, and why: Move "Update the plugin's pubspec.yaml" outside of "legacy apply" tab.

Issues fixed by this PR (if any): #13315

PRs or commits this PR depends on (if any):

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[gemini-code-assist]

Code Review

This pull request adds a closing tag to the documentation for migrating plugins to built-in Kotlin. A review comment points out that the parent component is also missing its closing tag, which is necessary to ensure the page renders correctly and follows the structure of similar documentation pages.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 4d82124):

https://flutter-docs-prod--pr13316-jmagman-patch-1-3gtfeftb.web.app