stellar-cli 21.3.0 (new formula)

[leighmcculloch]

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• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
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Add formula for stellar-cli 21.3.0 an accompanying tool to the existing stellar-core formula.

Close stellar/stellar-cli#1446

[carlocab]

Thanks for the PR! A few comments.

[carlocab]

Thanks!

[github-actions]

@chenrui333 has requested bottles to be published to this PR.