Add support for Ruby 3.1

[fujimotos]

Introduction

We are plannning to bundle fluent-plugin-windows-exporter in TD Agent,
starting from the next version (the release date will be March 2022).

Since TD Agent is now moving to Ruby 3.x, we also need to confirm that
this plugin works on Ruby 3.1 as well.

Task

• Add Ruby 3.1 to GitHub Actions CI.
  • Use matrix notation to test both on 2.7 and 3.1.
  • Here is an example
• Fix anything that are not compatible with Ruby 3.x.

Strategic priority

See fluent/fluentd#3585 for migration status.

[daipom]

@fujimotos
Thank you for the clear assignment.

https://github.com/fluent-plugins-nursery/fluent-plugin-windows-exporter/actions/runs/1845181374

I have added Ruby 3.1 in the add-ci-support-ruby31 branch
and all tests have passed in Ruby 3.1.

Would merging this into master complete this task?

[fujimotos]

@daipom Thank you! Marged via 6e3833d.

[daipom]

Thank you for merging!

[fujimotos]

Addendum: I documented the supported Ruby version in 8c9ad68.

[daipom]

Nice! Thank you!