Explicit ruby-version in RuboCop workflow is removed

setup-ruby reads from .ruby-version by default; the matrix
variable was redundant.

Author: AlfonsoUceda

.github/workflows/rubocop.yml

@@ -13,14 +13,12 @@ jobs:
       contents: read
       id-token: write
 
-
     steps:
       - uses: actions/checkout@v6
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
 
       - name: Lint Ruby code with RuboCop