Fix clippy lint caused by clap derive macro (#505)

A deprecated function was being called in the clap macro due to changes in their API. The issue is described here clap-rs/clap#3822.

server/svix-server/src/main.rs

@@ -21,7 +21,7 @@ struct Args {
     #[clap(subcommand)]
     command: Option<Commands>,
     /// Run database migrations before starting
-    #[clap(long)]
+    #[clap(long, value_parser)]
     run_migrations: bool,
 }