Use CircleCI matrix jobs to test with the latest version

[mishina2228]

This PR updates .circleci/config.yml to use CircleCI matrix jobs. It can make it easy to test with multiple Ruby and Kafka versions, and also can simplify duplicate statements. Here are the details:

• Upgrade to next-gen Docker convenience images The following message is shown in Circle CI job page.

  You’re using a deprecated Docker convenience image. Upgrade to a next-gen Docker convenience image.

  The link above states the following.

  Moving from a legacy to next-gen image requires a change to the namespace. All legacy images have a Docker namespace of circleci, while next-gen images have a Docker namespace of cimg. For example, migrating from the legacy Ruby or Python image to the respective next-gen image can be done as follows:

  circleci/ruby:2.3.0 → cimg/ruby:2.3.0 circleci/python:3.8.4 → cimg/python:3.8.4

  circleci/ruby:2.5.1-node is used now, but because of the following reasons, cimg/ruby:2.5 will be used.

  • cimg doesn't provide Ruby 2.5.1, and the latest patch version is 2.5.9.
  • Specify a minor release tag for the latest patch version to use. https://circleci.com/developer/images/image/cimg/ruby#image-tags
  • It seems that Node.js is not needed, so I omit the suffix -node.

  BTW, Ruby 2.5 has already reached EOL, but it's left as is in this PR.

• While unit tests are currently only run on Ruby 2.5, matrix is used so that we can add test for other Ruby versions later.

• There are two types of kafka images(provided by wurstmeister/bitnami), with different environment variables to pass. So I defined jobs for each(functional-with-wurstmeister/functional-with-bitnami).

• The workflow name was set for clarity. Note that the name has changed from the previous name. Before:

  Show image

  

  After:

  Show image

  

The CI status can be seen here. Functional tests fail randomly, but I believe this is not related with changes in this PR.