Add feature flag to introduce Claude 3.7 Sonnet

Add a feature flag to rails to control the new prompt version for Claude Sonnet 3.7 (added in gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2034 (merged))

Simply add a feature flag to control the prompt version for code generation and specify 1.0.2-dev as the prompt version to get the new Claude 3.7 Sonnet

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/521044