Change required ruby version after upgrading rubocop-graphql

[an-korn]

Currently checks are failed on current version. It's appeared after upgrading rubocop-graphql in https://github.com/datarockets/ruby-style/commit/cbc6697393240aa97099ffcd4631bf4bc43147fb to rubocop-graphql >= 1.2.0 which depends on Ruby >= 3.0.

Before you submit a pull request, please make sure you have to follow:

• [x] read and know items from the Contributing Guide
• [x] add a description of the problem you're trying to solve (short summary from related issue)
• [x] verified that cops are ordered by alphabet
• [x] add a note to the style guide docs (if it needs)
• [x] add a note to the changelog file
• [x] the commit message contains the number of the related issue (if it presents) and word Fix if this PR closes related issue
• [x] squash all commits before submitting to review