Resolve "[FF] use_duo_context_exclusion"

What does this MR do and why?

Removes the use_duo_context_exclusion feature flag and releases Duo Context Exclusion as GA

Related client-side LSP MR gitlab-org/editor-extensions/gitlab-lsp!2558 (merged)

Related AIGW MR gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!3903 (merged)

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Related to #548612