docs: add ActiveCollab to used-by.md (#1050)

Author: BojanRatkovic

versioned_docs/version-6.x/used-by.md

@@ -46,6 +46,7 @@ It's impossible to list every single app that uses React Navigation, but below a
 - [Tracker Network for Fortnite](https://apps.apple.com/us/app/tracker-network-for-fortnite/id1287696482)
 - [vrbo](https://www.vrbo.com/mobile/)
 - [Disprz](https://www.disprz.com/) ([iOS](https://apps.apple.com/us/app/disprz/id1458716803#?platform=iphone)) ([Android](https://play.google.com/store/apps/details?id=com.disprz&hl=en_IN&gl=US))
+- [ActiveCollab](https://activecollab.com/) ([iOS](https://apps.apple.com/us/app/activecollab-work-management/id1509421965)) ([Android](https://play.google.com/store/apps/details?id=com.activecollab.mobile))
 
 ## Your app?