Update Code Tasks (Slash Commands) to use Claude 3

groupcode creation created 3 code tasks for Duo Chat, refactoring, explaining code, and writing tests.

See: https://handbook.gitlab.com/handbook/engineering/development/dev/create/code-creation/engineering_overview/#code-tasks

We should make sure these are using the latest Claude 3 model to align with the rest of Chat

• ee/lib/gitlab/llm/chain/tools/refactor_code/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/write_tests/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/explain_code/prompts/anthropic.rb