Pin android and iOS platform versions

src/Android/Avalonia.Android/Avalonia.Android.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net7.0-android</TargetFramework>
+    <TargetFramework>net7.0-android33</TargetFramework>
     <SupportedOSPlatformVersion>21</SupportedOSPlatformVersion>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <MSBuildEnableWorkloadResolver>true</MSBuildEnableWorkloadResolver>

src/iOS/Avalonia.iOS/Avalonia.iOS.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net7.0-ios</TargetFramework>
+    <TargetFramework>net7.0-ios16.2</TargetFramework>
     <SupportedOSPlatformVersion>13.0</SupportedOSPlatformVersion>
     <MSBuildEnableWorkloadResolver>true</MSBuildEnableWorkloadResolver>
   </PropertyGroup>