[Php-Symfony] add text/plain and image/png endpoint in petstore.yaml

[gturri]

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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.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
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PR #21261 added support for endpoints with response of type text/plain or even image/png.

This commit adds such endpoints (and the subsequently generated samples) so that:

• the way those are supported is clearer (as it is now directly visible in the generated sample files)
• if a future commit impacts this part of the generation it will be easier to assess that impact

(nb: first time I'm editing an openapi specification in that repo, I hope I'm doing that commit correctly. Please let me know if I'm doing something wrong, or missing something)

Ping PHP committee: @jebentier @dkarlovi @mandrean @jfastnacht @ybelenko @renepardon