PHP 7.4 compatibility

[lenaorobei]

Steps to reproduce

See travis build results https://travis-ci.com/github/magento/magento-coding-standard/jobs/372726109

Expected result

Job passes without deprecation warning

Actual result

Job fails with Array and string offset access syntax with curly braces is deprecated

[m2-assistant]

Hi @lenaorobei. Thank you for your report.
To help us process this issue please make sure that you provided sufficient information.

Please, add a comment to assign the issue: @magento I am working on this

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