Tweak fill in merge request template prompt

What does this MR do and why?

Based on recent testing, there are instances that new sections are being added to the MR description being filled in by AI.

Following guidelines from https://cloud.google.com/vertex-ai/docs/generative-ai/learn/introduction-prompt-design, examples are added to the prompt and the instructions are now more concise.

This is behind the fill_in_mr_template feature flag.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415533 (closed)