Fix breaking change "Raw images on Web uses correct origin and colors"

[dkwingsmt]

This PR fixes https://docs.flutter.dev/release/breaking-changes/raw-images-on-web-uses-correct-origin-and-colors so that:

• The migration code in the original article now works on all browsers (instead of only on Chrome)
• The links are no longer broken.
• A related PR is added.

This PR should not be merged until flutter/engine#30343 is.

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.

[khanhnwin]

Looks like flutter/engine#30343 has landed. LGTM. Thanks @dkwingsmt!

[sfshaza2]

Thanks, @dkwingsmt!