Use 2.6 as the target RuboCop version

vinistock · hsbt · commit 0913639512f9 · 2023-06-03T10:42:22.000+09:00
diff --git a/.rubocop.yml b/.rubocop.yml
@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.5
+  TargetRubyVersion: 2.6
   DisabledByDefault: true
   Exclude:
     - rdoc.gemspec
