* Bump 3.1 branch to 3.1.6 (#1014)

Ruby 3.1.6 has been released:
https://www.ruby-lang.org/en/news/2024/05/29/ruby-3-1-6-released/

Bump 3.1 branch from 3.1.5 to 3.1.6:
ruby/ruby@v3_1_5...v3_1_6

There seems to be no change to the syntax.

Author: koic

.github/workflows/test.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["3.0.7", "3.1.5", "3.2.4", "3.3.1", "jruby-9.2"]
+        ruby: ["3.0.7", "3.1.6", "3.2.4", "3.3.1", "jruby-9.2"]
         test_command: ["bundle exec rake test"]
         include:
           - ruby: "head"

lib/parser/current.rb

@@ -93,7 +93,7 @@ def warn_syntax_deviation(feature, version)
     CurrentRuby = Ruby30
 
   when /^3\.1\./
-    current_version = '3.1.5'
+    current_version = '3.1.6'
     if RUBY_VERSION != current_version
       warn_syntax_deviation 'parser/ruby31', current_version
     end