Refactor code to use response.json() instead of response.text() in Jo…

[SpartanDeBoss]

…inRetroPage.jsx

Description

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Fixes # (issue)

How to test?

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Checklist

• I have performed a self-review of my own code
• I have followed the name conventions for CSS Classnames and filenames, Components names and filenames, Style filenames, if you are in doubt check the the project README.MD and here https://github.com/HackYourFuture-CPH/curriculum/blob/master/review/review-checklist.md
• I have commented my code, particularly in hard-to-understand areas, if you code was simple enough mark the box anyway
• I have made corresponding changes to the documentation, if you code was simple enough mark the box anyway
• This PR is ready to be merged and not breaking any other functionality

[SpartanDeBoss]

Hello @orhantoy,
About yesterday you mentioned that I could "push it". I wanted to follow up with you regarding the changes you made. It seems that it's not necessary to generate a code as any characters can get you to the next page. Could you please take a look at it for us? Thank you!