🔥 Remove flutter as a submodule

[AlexV525]

Description of what this PR is changing or adding, and why:
The PR removes the flutter repo as a submodule to avoid conflicts when building the website on different platforms. It was first introduced in #1257 but with no explanation so we cannot track back what's the purpose, but removing this should not cause difficulties in maintaining the website.

Additional info

• The build process is slower than the previous one because the checkout step needs an extra 3~5 minutes, depending on the network condition. See now and previous checks.

Issues fixed (partially) by this PR (if any):

• make setup fails with /usr/bin/env: ‘bash\r’: No such file or directory #6201
• Fix website local builds on Windows #6443
• [Makefile] Impossible to setup on Windows #8272

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[github-actions]

Visit the preview URL for this PR (updated for commit 9347ffe):

https://flutter-docs-prod--pr9307-feat-remove-flutter-4mo6emc3.web.app

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[AlexV525]

If we need flutter for local builds, we might consider adding it to the .gitignore too, and maybe with some more setup scripts.

[AlexV525]

With the change, I was able to run make test locally on my Windows setup. But still not able to run make up because some symbolic link issues between platforms.

[parlough]

Thanks for working on this @AlexV525!

I haven't taken a close look yet, but I'll give it some thought tonight :)

Currently we benefit from dependabot updating the submodule for each stable Flutter release. This allows us to explicitly move to a new stable version of Flutter, rather than the build using whatever the stable branch happens to be at, potentially breaking the build with updates. In practice, this isn't usually an issue since we prepare for new stable builds in advance.

[AlexV525]

This allows us to explicitly move to a new stable version of Flutter, rather than the build using whatever the stable branch happens to be at, potentially breaking the build with updates.

It can be done if we wrote the current ref we're using into configurations and use some dependabot mechanism to upgrade as I imagine.

[parlough]

Sorry about the delay on reviewing this. Thank you!