Relax reverse_markdown dependency to Support v3

The `reverse_markdown` library has been updated to version 3. The only
breaking change in this version is the removal of support for
Ruby versions <= 2.6. 

This commit relaxes the dependency to allow versions up to `< 4`,
ensuring compatibility with the latest `reverse_markdown` while
maintaining support for Ruby 2.6.

Closes castwide#728

Note: The CI failure is unrelated to this change and is being tracked in
issue castwide#726.

solargraph.gemspec

@@ -33,7 +33,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'kramdown-parser-gfm', '~> 1.1'
   s.add_runtime_dependency 'parser', '~> 3.0'
   s.add_runtime_dependency 'rbs', '~> 2.0'
-  s.add_runtime_dependency 'reverse_markdown', '~> 2.0'
+  s.add_runtime_dependency 'reverse_markdown', '>= 2.0', '< 4'
   s.add_runtime_dependency 'rubocop', '~> 1.38'
   s.add_runtime_dependency 'thor', '~> 1.0'
   s.add_runtime_dependency 'tilt', '~> 2.0'