[dotnet] Update bazel build to recognize Support as nullable-annotated and langversion

Author: RenderMichael

dotnet/src/support/BUILD.bazel

@@ -35,6 +35,8 @@ csharp_library(
         "UI/*.cs",
     ]) + [":assembly-info"],
     out = "WebDriver.Support",
+    langversion = "12.0",
+    nullability = "annotations",
     target_frameworks = [
         "netstandard2.0",
     ],
@@ -74,6 +76,8 @@ csharp_library(
     ]) + [":assembly-info"],
     out = "WebDriver.Support.StrongNamed",
     keyfile = "//dotnet:WebDriver.snk",
+    langversion = "12.0",
+    nullability = "annotations",
     target_frameworks = [
         "netstandard2.0",
     ],