C#: Default subtypes to true.

[michaelnebel]

No description provided.

[github-actions]

⚠️ The head of this PR and the base branch were compared for differences in the framework coverage reports. The generated reports are available in the artifacts of this workflow run. The differences will be picked up by the nightly job after the PR gets merged.

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csharp

Generated file changes for csharp

• Changes to framework-coverage-csharp.rst:

-    System,"``System.*``, ``System``",47,10818,54,5
+    System,"``System.*``, ``System``",47,10817,54,5
-    Totals,,104,12893,396,5
+    Totals,,104,12892,396,5

• Changes to framework-coverage-csharp.csv:

- System,54,47,10818,,6,5,5,,,4,1,,33,2,,6,15,17,4,3,,5511,5307
+ System,54,47,10817,,6,5,5,,,4,1,,33,2,,6,15,17,4,3,,5511,5306