Reset Notifications min version to 10240 (#3582)

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## Partially addresses #3581 issue 2
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## PR Type
What kind of change does this PR introduce?
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- Build or CI related changes


## What is the current behavior?
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Min version of UWP package in master incorrectly got bumped to 17763

![image](https://user-images.githubusercontent.com/13246069/100283018-86fbb080-2f21-11eb-8622-78a7707d2599.png)


## What is the new behavior?
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Min version of UWP package is 10240. Had to make sure the DefaultTargetMinVersion and TargetMinVersion properties were set in the Notifications project (so the overall default of 17763 didn't get used).

![image](https://user-images.githubusercontent.com/13246069/100290310-e2cd3600-2f2f-11eb-9fd8-e05a1263dcf4.png)



## PR Checklist

Please check if your PR fulfills the following requirements:

- [x] Tested code with current [supported SDKs](../readme.md#supported)
- [x] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->
- [x] Sample in sample app has been added / updated (for bug fixes / features)
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- [x] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [x] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [x] Contains **NO** breaking changes

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## Other information

Microsoft.Toolkit.Uwp.Notifications/Microsoft.Toolkit.Uwp.Notifications.csproj

@@ -15,6 +15,8 @@
     </Description>
     <PackageTags>notifications win10 windows 10 tile tiles toast toasts badge xml uwp c# csharp c++</PackageTags>
     <ExtrasImplicitPlatformPackageIsPrivate Condition=" '$(TargetFramework)' == 'native' ">true</ExtrasImplicitPlatformPackageIsPrivate>
+    <DefaultTargetPlatformMinVersion>10240</DefaultTargetPlatformMinVersion>
+    <TargetPlatformMinVersion>10.0.10240.0</TargetPlatformMinVersion>
   </PropertyGroup>
 
   <Choose>