Require RuboCop 1.30+ as runtime dependency

This PR makes RuboCop Performance require RuboCop 1.30+ to fix some errors:
https://github.com/rubocop/rubocop-performance/actions/runs/6966652446/job/18957154150

It introduces CI for the oldest RuboCop version still supported,
implementing a workflow for regression testing that is similar to the one found a
rubocop/rubocop-rails@c9acb7a.

.github/workflows/test.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  oldest_supported_rubocop:
+    runs-on: ubuntu-latest
+    name: The oldest supported RuboCop version
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use the oldest supported RuboCop
+        run: |
+          sed -e "/gem 'rubocop', github: 'rubocop\/rubocop'/d" \
+              -e "/gem 'rubocop-rspec',/d" -i Gemfile
+          cat << EOF > Gemfile.local
+            gem 'rubocop', '1.30.0' # Specify the oldest supported RuboCop version
+          EOF
+      - name: set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+          bundler-cache: true
+      - name: spec
+        run: bundle exec rake spec

changelog/change_require_rubocop_1_33_as_a_runtime_dependency.md

@@ -0,0 +1 @@
+* [#388](https://github.com/rubocop/rubocop-performance/pull/388): Require RuboCop 1.30+ as runtime dependency. ([@koic][])

lib/rubocop/cop/performance/redundant_merge.rb

@@ -130,7 +130,9 @@ def to_assignments(receiver, pairs)
         end
 
         def rewrite_with_modifier(node, parent, new_source)
-          indent = ' ' * configured_indentation_width
+          # FIXME: `|| 2` can be removed when support is limited to RuboCop 1.44 or higher.
+          # https://github.com/rubocop/rubocop/commit/02d1e5b
+          indent = ' ' * (configured_indentation_width || 2)
           padding = "\n#{indent + leading_spaces(node)}"
           new_source.gsub!("\n", padding)
 

rubocop-performance.gemspec

@@ -30,6 +30,6 @@ Gem::Specification.new do |s|
     'rubygems_mfa_required' => 'true'
   }
 
-  s.add_runtime_dependency('rubocop', '>= 1.7.0', '< 2.0')
+  s.add_runtime_dependency('rubocop', '>= 1.30.0', '< 2.0')
   s.add_runtime_dependency('rubocop-ast', '>= 1.30.0', '< 2.0')
 end