Pin rubocop-ast development gem due to new dep on prism (#17407) (#17428)

The rubocop-ast gem just introduced a new dependency on prism.
 - https://rubygems.org/gems/rubocop-ast/versions/1.43.0

In our install default gem rake task we are seeing issues trying to build native
extensions. I see that in upstream jruby they are seeing a similar problem (at
least it is the same failure mode jruby/jruby#8415

This commit pins rubocop-ast to 1.42.0 which is the last version that did not
have an explicit prism dependency.

(cherry picked from commit 6de59f2)

Co-authored-by: Cas Donoghue <[email protected]>

Author: mergify[bot]

Gemfile.template

@@ -25,6 +25,8 @@ gem "stud", "~> 0.0.22", :group => :build
 gem "fileutils", "~> 1.7"
 
 gem "rubocop", :group => :development
+# rubocop-ast 1.43.0 carries a dep on `prism` which requires native c extensions
+gem 'rubocop-ast', '= 1.42.0', :group => :development
 gem "belzebuth", :group => :development
 gem "benchmark-ips", :group => :development
 gem "ci_reporter_rspec", "~> 1", :group => :development