Enable summarize_merge_request_claude_3_7_sonnet feature flag by default

What does this MR do and why?

• Given the work done on this issue: Self-managed support and access control (unit p... (#497503 - closed), the summarize merge request feature is available for self-managed instance.
• This MR enables the summarize_merge_request_claude_3_7_sonnet feature flag by default to ensure self-managed instances also get to use the latest AI model, Claude 3.7 Sonnet.
• Additional context: #521034 (comment 2433225647)

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Related to #521034 (closed)