Fix supported Ruby version to be only 3.1.x

[kudojp]

Why

Current implementation of the scheduler cannot be used in Ruby ver3.1.x, because the scheduler of this gem accepts only 3 arguments (src) on #io_read.

• In Ruby 3.0.x, io_read is called with 4 arguments (not documented, src)
• In Ruby 3.1.x, io_read is called with 3 arguments (doc, src)
• In Ruby 3.2.x, io_read is called with 3 arguments (doc is wrong, src) - Not added to CI testing yet

What

• Specify spec.required_ruby_version = "~> 3.1.0" in gemspec
• Remove CI testing with Ruby ver 3.0.

---

📝 Why this was not noticed so far.

The CI testing with Ruby 3.0 began to fail suddenly on some time between Dec 13, 2022 and Jul 4, 2023.
I will explain the reason of this.

jobs:
  test:
    strategy:
      matrix:
        os: [ubuntu-latest, macos-latest]
        ruby: [3.0, 3.1]
    runs-on: ${{ matrix.os }}
    steps:
      - uses: ruby/setup-ruby@v1
        with:
          ruby-version: ${{ matrix.ruby }}

On Dec 13, 2022 ✅

CI testing was triggered. At this time, the latest ruby was 3.1.x.
3.0 was interpreted as 3, and the latest version of 3.x (=3.1.x) was used.
(ref: actions/runner#849 ruby/setup-ruby#252).

Since #io_read takes 3 arguments in Ruby3.1, the CI testing did not fail.

On Dec 25, 2022

Ruby 3.2 was released. At this time, the latest ruby became 3.2.x.

On Jul 4, 2023 ❌

CI testing was triggered. At this time, the latest ruby was 3.2.x.
3.0 was interpreted as 3, and the latest version of 3.x (= 3.2.x), was used.

Since #io_read takes 4 arguments in Ruby3.2, the CI testing failed.

[codecov-commenter]

Codecov Report

Merging #32 (463e3b2) into main (cfc404d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #32   +/-   ##
=======================================
  Coverage   66.96%   66.96%           
=======================================
  Files           3        3           
  Lines         112      112           
=======================================
  Hits           75       75           
  Misses         37       37           

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