Update rdoc requirement from ~> 3.12 to ~> 6.3

Updates the requirements on [rdoc](https://github.com/ruby/rdoc) to permit the latest version.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v3.12.2...v6.3.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -1,19 +1,16 @@
 PATH
   remote: .
   specs:
-    bcrypt (3.1.11)
+    bcrypt (3.1.12.pre)
 
 GEM
   remote: https://rubygems.org/
   specs:
     diff-lcs (1.2.5)
-    json (1.8.3)
-    json (1.8.3-java)
     rake (10.4.2)
     rake-compiler (0.9.5)
       rake
-    rdoc (3.12.2)
-      json (~> 1.4)
+    rdoc (6.3.1)
     rspec (3.3.0)
       rspec-core (~> 3.3.0)
       rspec-expectations (~> 3.3.0)
@@ -37,7 +34,7 @@ PLATFORMS
 DEPENDENCIES
   bcrypt!
   rake-compiler (~> 0.9.2)
-  rdoc (~> 3.12)
+  rdoc (~> 6.3)
   rspec (>= 3)
 
 BUNDLED WITH

bcrypt.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'rake-compiler', '~> 0.9.2'
   s.add_development_dependency 'rspec', '>= 3'
-  s.add_development_dependency 'rdoc', '~> 3.12'
+  s.add_development_dependency 'rdoc', '~> 6.3'
 
   s.has_rdoc = true
   s.rdoc_options += ['--title', 'bcrypt-ruby', '--line-numbers', '--inline-source', '--main', 'README.md']