Update Nullable annotation library

Switched from JetBrains to jspecify Nullable annotation in CommanderVersionChecker. This change aligns with the project's new dependency guidelines and ensures consistent annotation usage.

paper/src/main/java/net/thenextlvl/commander/paper/version/CommanderVersionChecker.java

@@ -4,8 +4,8 @@
 import core.version.SemanticVersion;
 import lombok.Getter;
 import net.thenextlvl.commander.paper.CommanderPlugin;
-import org.jetbrains.annotations.Nullable;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 import java.util.Objects;