[PHP] \GuzzleHttp\json_encode is deprecated, use \GuzzleHttp\Utils::jsonEncode instead

[lddsb]

• perf: \GuzzleHttp\json_encode is deprecated, use \GuzzleHttp\Utils::jsonEncode instead

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

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[wing328]

cc @jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)

[wing328]

tests passed via https://app.travis-ci.com/github/OpenAPITools/openapi-generator/builds/259194790