CI: Add Ruby 3.2 and drop 2.6

Signed-off-by: Takuro Ashie <[email protected]>

Author: ashie

.github/workflows/linux.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '3.1', '3.0', '2.7', '2.6' ]
+        ruby: [ '3.2', '3.1', '3.0', '2.7' ]
         os:
           - ubuntu-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}