Revert "Lock rdoc gem to v6.3.x to avoid Psych 4.0"

Reason: No longer necessary since CI has stopped testing on Ruby 2.5

This reverts 58de2b2
This reverts 96c3314

Author: ashmaroli

History.markdown

@@ -6,7 +6,6 @@
 
 ### Development Fixes
 
-  * Lock `rdoc` gem to v6.3.x to avoid Psych 4.0 (#491)
   * Bump dependencies and CI Ruby versions (#493)
 
 ## 0.21.0 / 2021-10-31

jekyll-import.gemspec

@@ -30,7 +30,7 @@ Gem::Specification.new do |s|
   # development dependencies
   s.add_development_dependency("bundler")
   s.add_development_dependency("rake", "~> 13.0")
-  s.add_development_dependency("rdoc", "~> 6.3.0")
+  s.add_development_dependency("rdoc", "~> 6.0")
 
   # test dependencies:
   s.add_development_dependency("redgreen", "~> 1.2")