[dotnet] Bump min macOS version in XIB to 10.15.

dotnet/Templates/Microsoft.macOS.Templates/macos-viewcontroller/ViewController1.xib

@@ -1,7 +1,8 @@
-<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="21225" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
+<?xml version="1.0" encoding="UTF-8"?>
+<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="21507" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
     <dependencies>
-        <deployment version="101400" identifier="macosx"/>
-        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="21225"/>
+        <deployment version="101500" identifier="macosx"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="21507"/>
         <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
     </dependencies>
     <objects>