Update hashtags in release announcement template to include C# and .NET

Signed-off-by: Yuki Kishimoto <[email protected]>

Author: yukibtc

contrib/release/ANNOUNCEMENT_TEMPLATE.txt

@@ -17,4 +17,4 @@ Thanks to all contributors!
 https://rust-nostr.org
 https://rust-nostr.org/donate
 
-#rustnostr #nostr #rustlang #programming #rust #python #javascript #kotlin #swift #flutter
+#rustnostr #nostr #rustlang #programming #rust #python #csharp #dotnet #javascript #kotlin #swift #flutter