Set db compat version

database/upgrades/00-latest-revision.sql

@@ -1,4 +1,4 @@
--- v0 -> v18: Latest revision
+-- v0 -> v18 (compatible with v15+): Latest revision
 
 CREATE TABLE guild (
     dcid       TEXT PRIMARY KEY,

database/upgrades/18-extra-ghost-metadata.sql

@@ -1,4 +1,4 @@
--- v18: Store additional metadata for ghosts
+-- v18 (compatible with v15+): Store additional metadata for ghosts
 ALTER TABLE puppet ADD COLUMN username TEXT NOT NULL DEFAULT '';
 ALTER TABLE puppet ADD COLUMN discriminator TEXT NOT NULL DEFAULT '';
 ALTER TABLE puppet ADD COLUMN is_bot BOOLEAN NOT NULL DEFAULT false;