Remove prompt indentation

What does this MR do and why?

Remove unnecessary prompt indentations, by using squiggly heredoc.

May improve result since Claude prompts typically ends with "\n\nAssistant", and indentation breaks this.

How to set up and validate locally

Gitlab::Llm::Templates::CategorizeQuestion.new(User.first, {question: "What's my name?"}).to_prompt

MR acceptance checklist

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