.Net: Update nuget version to 0.20 (microsoft#2515)

### Motivation and Context

Updated SK version in NuGet to 0.20 as new breaking changes were
introduced.

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### Description

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### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: dmytrostruk

dotnet/nuget/nuget-package.props

@@ -1,7 +1,7 @@
 <Project>
   <PropertyGroup>
     <!-- Central version prefix - applies to all nuget packages. -->
-    <Version>0.19</Version>
+    <Version>0.20</Version>
 
     <Configurations>Debug;Release;Publish</Configurations>
     <IsPackable>true</IsPackable>