Pin childprocess gem to major version 4 (#15758)

This commit pins the `childprocess` gem to version `4` since version `5.0.0` of
enkessler/childprocess#175 seems to have broken JRuby support for spawning.

Closes #15757

Co-authored-by: Andrea Selva <selva.andre@gmail.com>
Co-authored-by: João Duarte <jsvd@users.noreply.github.com>
(cherry picked from commit 9f1d55c)

qa/integration/integration_tests.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |s|
 
   # Gem dependencies
   s.add_development_dependency 'elasticsearch'
-  s.add_development_dependency 'childprocess'
+  s.add_development_dependency 'childprocess', '~> 4' # https://github.com/enkessler/childprocess/pull/175 landed in 5.0.0 and seems to have broken JRuby support for spawning processes. sticking to 4.x.
   s.add_development_dependency 'rspec-wait'
   s.add_development_dependency 'manticore'
   s.add_development_dependency 'stud', '~> 0.0.22'