fix(share_plus): remove deprecated UIApplication.keyWindow fallback on iOS

[ledexsoft]

Fixes a compile warning surfaced when building the iOS pod with Flutter 3.44.8 / Xcode 26 (iOS 26 SDK):

warning: 'keyWindow' is deprecated: first deprecated in iOS 13.0

Cause

FPPSharePlusPlugin.m keeps a legacy fallback branch that reads [UIApplication sharedApplication].keyWindow for iOS < 13. That branch still compiles (and warns) because the CocoaPods podspec declares s.platform = :ios, '12.0', even though:

• the SwiftPM manifest (ios/share_plus/Package.swift) already requires iOS 13.0 (raised in fix(share_plus): raise Apple platform minimums in SPM manifests to match FlutterFramework #3923 to match FlutterFramework), and
• the Flutter iOS engine itself now has MinimumOSVersion 13.0.

Fix

• Remove the @available(iOS 13, *) conditional and the deprecated keyWindow fallback, keeping only the iOS 13+ scene-based lookup (connectedScenes → UIWindowScene.windows → isKeyWindow), which is the code already used on iOS 13+.
• Align the CocoaPods deployment target with SwiftPM by raising the podspec to s.platform = :ios, '13.0'.

This supersedes the earlier #1157, which introduced the scene-based lookup but had to keep the deprecated fallback because the podspec still targeted iOS 12.

Verification

• clang -fsyntax-only against the iOS 26 simulator SDK (deployment target 13.0) passes with no keyWindow reference remaining.
• No behavior change on iOS 13+; the removed branch was only reachable on iOS 12, which is below the Flutter engine minimum.

Detected with Flutter 3.44.8 / Xcode 26 (iOS 26 SDK). Note: per plus_plugins convention, CHANGELOG/version bumps are handled by the maintainers' release flow, so this PR touches only the plugin sources.