Refactor: Centralize Noise Field Filter

refactor: extract field key filter into shared static method

Moved duplicate noise-stripping logic into Flamingo_Export_Core::filter_field_keys() so the same filter applies consistently across all export modes and AJAX responses.

• All Fields mode now strips CleanTalk, reCAPTCHA, CF7 internals and synthetic keys, matching Channel Fields behavior
• ajax_channel_fields() replaced inline array_filter with single call to filter_field_keys()
• build_rows() applies filter_field_keys() when collecting keys for both all and channel modes
• Single source of truth: update the filter once to affect all export paths
• Add ChangeLog for Git Version