Upgrade Ruby to 2.7 or 3.1

Currently our Docker images uses Ruby 2.6:

Line 45 in 341fa81

FROM ruby:2.6-slim-buster

EOL date of Ruby 2.6 is 2022-03-31:https://www.ruby-lang.org/en/downloads/branches/
We need to upgrade Ruby by the date.

Because Ruby 3.0 has some known problems (that are solved in 3.1), I want to skip it.

We have still several known issues with Ruby 3.1 on Windows: fluent/fluentd#3585
So I'll choose 2.7 for a while.

And more, buster reaches EOL at 2022-08, ruby:2.7-slim-bullseye is recommended.

Working on windows side.
#309
#310

Fixed other base images
#311

PRs were all merged.
It will be shipped when v1.14.5 was ready.

Done