Docs: change c++ version in cmake example from 17 to 20

[ruslan-ilesik]

Documentation change checklist

• My documentation changes follow the docs style guide and any code examples follow the coding style guide.
• I tested that my change works before raising the PR (via running doxygen, and testing examples).
• I have not moved any existing pages or changed any existing URLs without strong justification as to why.
• I have not generated content using AI or a desktop utility such as grammarly.

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