Bump org.spigotmc:spigot-api from 1.20.6-R0.1-SNAPSHOT to 1.21-R0.1-SNAPSHOT

Dependabot couldn't find the original pull request head commit, 2dc9361.

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 jetbrains-annotations = "24.1.0"
-spigot = "1.20.1-R0.1-SNAPSHOT"
+spigot = "1.21.4-R0.1-SNAPSHOT"
 paperSpigot = "1.20.1-R0.1-SNAPSHOT"
 junit = "5.10.1"
 mockito = "4.11.0"