fix(ios): bump minimum deployment target to 13.0 to match Mixpanel-swift 6.x

[TheoGermain]

Problem

Mixpanel-swift 6.x requires iOS 13.0 as minimum deployment target. The podspec still declared 12.0, causing CocoaPods to error during pod install:

Error: The pod "Mixpanel-swift" required by the plugin "mixpanel_flutter" requires
a higher minimum iOS deployment version than the plugin's reported minimum version.
To build, remove the plugin "mixpanel_flutter", or contact the plugin's developers
for assistance.
Error running pod install

Fix

Bump s.platform = :ios, '13.0' in ios/mixpanel_flutter.podspec to match the actual minimum required by Mixpanel-swift 6.x.

Notes

• iOS 13.0 is a reasonable minimum in 2025 (99%+ of active devices)
• This aligns the declared minimum with what Mixpanel-swift already requires transitively