docs: clarify FlutterLoader.loadEntrypoint deprecation

[lamek]

Fixes #12184

Clarifies the deprecation of FlutterLoader.loadEntrypoint in favor of _flutter.loader.load() under a new subsection in the Common warnings section of the web initialization documentation.

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[gemini-code-assist]

Code Review

This pull request updates the web initialization documentation to include a new section addressing the deprecation of FlutterLoader.loadEntrypoint in favor of FlutterLoader.load(). The reviewer suggested adding a simpler alternative for users who have not customized their web/index.html file, which is to replace the old inline script block with the standard <script src="flutter_bootstrap.js" async></script> tag.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 00d76ae):

https://flutter-docs-prod--docs-pr13858-fix-12184-goxi9oet.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 00d76ae):

https://flutter-dev-230821--www-pr13858-fix-12184-5g50g4so.web.app