Add flutter 1.20.2

[andreiborisov]

After making all changes to a cask, verify:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• There are no open pull requests for the same update.
• The submission is for a stable version or documented exception.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• brew cask audit --new-cask {{cask_file}} worked successfully.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked the cask was not already refused.
• Checked the cask is submitted to the correct repo.

[andreiborisov]

This PR is updated version of #87344 to speed things up a bit since the original author is not available