Docs: Fix Duo analytics availability details and add missing prerequisites

What does this MR do?

Fixes several documentation gaps in the Duo and SDLC trends GraphQL API and dashboard pages, discovered during source code verification of the analytics authorization and resolver logic.

GraphQL API doc (doc/api/graphql/duo_and_sdlc_trends.md):

• Add version note: AiUsageData required the Duo Enterprise add-on prior to 18.7; on earlier versions with Duo Pro, the endpoint returns null without an error message
• Fix AiUserMetrics and AiMetrics offering badges to include GitLab Self-Managed (source code has no offering restriction)
• Document the 1-month maximum date range enforced by UsageEventsResolver#validate_params! on all and codeSuggestionEvents
• Document that Chat events do not populate the extras field

Dashboard doc (doc/user/analytics/duo_and_sdlc_trends.md):

• Add Reporter role and top-level group requirement to prerequisites

Related issues

Closes #591316 Closes #591317 Closes #591319 Closes #591336 Related https://gitlab.com/gitlab-org/gitlab/-/issues/548549 Related https://gitlab.com/gitlab-org/gitlab/-/issues/580174

Author's checklist

• Follow the Documentation process
• Follow the Style Guide
• Ensure that the product tier is correct

AI-Generated Content Disclosure: This MR contains code generated with assistance from GitLab Duo and OpenCode. The output has been reviewed for correctness, tested, and validated against project requirements per GitLab's AI contribution guidelines.