[dotnet] Finish nullability annotations on Support package (#15089)

Author: RenderMichael

dotnet/src/support/BUILD.bazel

@@ -36,7 +36,7 @@ csharp_library(
     ]) + [":assembly-info"],
     out = "WebDriver.Support",
     langversion = "12.0",
-    nullable = "annotations",
+    nullable = "enable",
     target_frameworks = [
         "netstandard2.0",
     ],
@@ -77,7 +77,7 @@ csharp_library(
     out = "WebDriver.Support.StrongNamed",
     keyfile = "//dotnet:WebDriver.snk",
     langversion = "12.0",
-    nullable = "annotations",
+    nullable = "enable",
     target_frameworks = [
         "netstandard2.0",
     ],