fix(deps): update rust crate serenity to 0.12.0

Author: renovate[bot]

Cargo.toml

@@ -14,7 +14,7 @@ publish = false
 
 [dependencies]
 # Discord
-serenity = "0.10.8"
+serenity = "0.12.0"
 
 # IO
 tokio = { version = "1.6.1", features = ["macros", "rt-multi-thread"] }