Changed benchmark target to Ruby latest (#236)

Ruby 3.4 has been released, we will change our benchmark target to Ruby
latest(3.4).

Co-authored-by: Sutou Kouhei <[email protected]>

Author: naitoh

.github/workflows/benchmark.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby-version:
-          - '3.3'
+          - 'ruby'
         runs-on:
           - ubuntu-latest
     runs-on: ${{ matrix.runs-on }}