According to `reverse_markdown` changelog, the only breaking change is
that it removes support for Ruby <= 2.6

Since this library still supports Ruby 2.6, the dependency has been
relaxed to also accept `< 4`

Close castwide#728

According to `reverse_markdown` changelog, the only breaking change is
that it removes support for Ruby <= 2.6

Since this library still supports Ruby 2.6, the dependency has been
relaxed to also accept `< 4`

Close castwide#728

Note: CI failure does not depend by this change and is tracked in castwide#726

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.