CI: Update ruby 2.4 -> 2.6 to fix CircleCI Packagecloud deploy step

[arm4b]

CircleCI started to fail on deploy step (https://app.circleci.com/pipelines/github/StackStorm/st2/4125/workflows/d62a5fd6-42c3-484f-acf7-5ba525ee22b9/jobs/15902)

with the following error:

ERROR:  Error installing package_cloud:
        The last version of thor (~> 1.2) to support your Ruby & RubyGems was 1.2.2. Try installing it with `gem install thor -v 1.2.2` and then running the current command again
        thor requires Ruby version >= 2.6.0. The current ruby version is 2.4.10.364.

Sync CircleCI config with st2web (https://github.com/StackStorm/st2web/blob/ba85bf2a495056ba4e1e22b8727a84baf4493c48/.circleci/config.yml#L153 config) with ruby:2.6 where deployment worked OK.