fix: don't delete rows from messages table when the associated member is deleted

PluralKit.Core/Database/Migrations/24.sql

@@ -0,0 +1,10 @@
+-- schema version 24
+-- don't drop message rows when system/member are deleted
+
+alter table messages alter column member drop not null;
+alter table messages drop contstraint messages_member_fkey;
+alter table messages
+    add constraint messages_member_fkey
+    foreign key (member) references members(id) on delete set null;
+
+update info set schema_version = 24;