Migrate Instant Models to Claude 3

Description

Following the successful migration of Chat "instant" models to Claude 3 Haiku, we aim to update the remaining use-cases that still rely on the DEFAULT_INSTANT_MODEL configuration to use Claude 3. This update will enhance performance and maintain consistency across our AI-powered features.

Current Status:

• Chat "instant" models have already been migrated to Claude 3 Haiku and are now the default behavior.
• Remaining use-cases using DEFAULT_INSTANT_MODEL:
  1. Generate issue description -> Claude Haiku
  2. Summarize comments (for an issue) -> Claude 3.5 Sonnet