Update usage for file name

What does this MR do?

Updates the usage of 'file name' (two words) to 'filename' one word, as per the word list.

Changes to usage in GraphQL docs will be in a separate MR.

Related issues

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ee/config/audit_events/types/group_push_rules_file_name_regex_updated.yml

 ---
 name: group_push_rules_file_name_regex_updated
-description: Event triggered when a groups push rules settings is changed for file name regex.
+description: Event triggered when a group's push rules settings is changed for filename regex.
 introduced_by_issue: https://gitlab.com/gitlab-org/gitlab/-/issues/369344
 introduced_by_mr: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/105791
 feature_category: source_code_management