fix(plugins) Mixpanel plugin wrong EU URL

[EugeneHlushko]

Summary
Fixes Mixpanel EU data residency routing in the React Native destination plugin. When Enable European Union Endpoint is on in Segment, the plugin now initializes mixpanel-react-native against https://api-eu.mixpanel.com instead of the broken api.eu.mixpanel.com hostname and the previous US-default-then-setServerURL() flow as per docs.

Changes

• MixpanelPlugin: correct EU_SERVER from api.eu.mixpanel.com → https://api-eu.mixpanel.com (Mixpanel’s documented EU ingestion endpoint; the old hostname does not resolve).
• MixpanelPlugin: add exported US_SERVER (https://api.mixpanel.com) and pass the chosen endpoint into mixpanel.init(undefined, undefined, serverURL) so the native SDK starts on the right region from the first request.
• MixpanelPlugin: remove post-init setServerURL() — avoids initializing against the US default and retargeting afterward (race with early lifecycle events when trackAppLifecycleEvents is enabled).

Why
Apps using the Mixpanel device-mode plugin with EU data residency enabled in Segment were not reliably sending events to Mixpanel’s EU ingestion API.