Update reverse_markdown requirement from ~> 2.1 to >= 2.1, < 4.0

Updates the requirements on [reverse_markdown](https://github.com/xijo/reverse_markdown) to permit the latest version.
- [Changelog](https://github.com/xijo/reverse_markdown/blob/master/CHANGELOG.md)
- [Commits](xijo/reverse_markdown@v2.1.1...v3.0.0)

---
updated-dependencies:
- dependency-name: reverse_markdown
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -6,7 +6,7 @@ PATH
       licensee (~> 9.16)
       parallel (~> 1.22)
       pathname-common_prefix (~> 0.0.1)
-      reverse_markdown (~> 2.1)
+      reverse_markdown (>= 2.1, < 4.0)
       ruby-xxHash (~> 0.4.0)
       thor (~> 1.2)
       tomlrb (~> 2.0)
@@ -75,7 +75,7 @@ GEM
     rainbow (3.1.1)
     rake (13.2.1)
     regexp_parser (2.6.2)
-    reverse_markdown (2.1.1)
+    reverse_markdown (3.0.0)
       nokogiri
     rexml (3.3.9)
     rubocop (1.45.1)

licensed.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "tomlrb", "~> 2.0"
   spec.add_dependency "ruby-xxHash", "~> 0.4.0"
   spec.add_dependency "parallel", "~> 1.22"
-  spec.add_dependency "reverse_markdown", "~> 2.1"
+  spec.add_dependency "reverse_markdown", ">= 2.1", "< 4.0"
   spec.add_dependency "json", "~> 2.6"
 
   spec.add_development_dependency "rake", "~> 13.0"