ci: drop older than 2.7

It is hard to keep maintaining CI for older platforms now.

e.g. ruby/setup-ruby explicitly drop support for CRuby < 2.6 and
so on.

  Either use a newer Ruby version or use a macOS image running on amd64, e.g., macos-15-intel.
  Note that GitHub changed the meaning of macos-latest from macos-12 (amd64) to macos-14 (arm64):
  https://github.blog/changelog/2024-04-01-macos-14-sonoma-is-generally-available-and-the-latest-macos-runner-image/

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/linux.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
+        ruby: [ '2.7', '3.0' ]
         os:
           - ubuntu-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}

.github/workflows/macos.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
+        ruby: [ '2.7', '3.0' ]
         os:
           - macOS-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}

.github/workflows/windows.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.5', '2.6', '2.7', '3.0' ]
+        ruby: [ '2.7', '3.0' ]
         os:
           - windows-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}