Upgrade steep typechecker to version 1.6.0

**What does this PR do?**

Upgrade the steep typechecker to version 1.6.0.

**Motivation:**

The steep typechecker is under very active development so there's a
number of improvements to be picked up, see
https://github.com/soutaro/steep/wiki/Release-Note-1.6 .

In particular, I'm very interested in improvements to error messages,
since I still think that's a particular weak point for steep.

**Additional Notes:**

N/A

**How to test the change?**

Validate that CI is still green.

Gemfile

@@ -113,7 +113,7 @@ end
 group :check do
   if RUBY_VERSION >= '3.0.0' && RUBY_PLATFORM != 'java'
     gem 'rbs', '~> 3.2.0', require: false
-    gem 'steep', '~> 1.5.3', require: false
+    gem 'steep', '~> 1.6.0', require: false
   end
 end