fix redcarpet warning

See https://github.com/vmg/redcarpet/blob/master/CHANGELOG.md#version-360.

From https://github.com/handshake-org/handshake-org.github.io/actions/runs/4471682840/jobs/7856884549:
```
/home/runner/work/handshake-org.github.io/handshake-org.github.io/build/vendor/ruby/3.2.0/gems/tilt-2.0.11/lib/tilt/redcarpet.rb:64: warning: undefining the allocator of T_DATA class Redcarpet::Markdown
```

Author: handshake-enthusiast

src/api-docs/Gemfile

@@ -8,7 +8,7 @@ gem 'middleman-autoprefixer', '~> 2.7.0'
 gem 'middleman-livereload', '~> 3.4.7'
 gem 'middleman-sprockets', '~> 4.1.1'
 gem 'rouge', '~> 3.28.0'
-gem 'redcarpet', '~> 3.5.1'
+gem 'redcarpet'
 gem 'nokogiri', '~> 1.13.4'
 gem 'rack', '~> 2.2.3'
 gem 'webrick', '~> 1.7.0'

src/api-docs/Gemfile.lock

@@ -108,7 +108,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    redcarpet (3.5.1)
+    redcarpet (3.6.0)
     rexml (3.2.5)
     rouge (3.28.0)
     sassc (2.4.0)
@@ -139,7 +139,7 @@ DEPENDENCIES
   middleman-syntax (~> 3.2.0)
   nokogiri (~> 1.13.4)
   rack (~> 2.2.3)
-  redcarpet (~> 3.5.1)
+  redcarpet
   rouge (~> 3.28.0)
   webrick (~> 1.7.0)