Just replace the table definition with the one from full_schema 16

synapse/storage/schema/delta/15/v15.sql

@@ -3,21 +3,20 @@
 CREATE TABLE IF NOT EXISTS pushers2 (
   id BIGINT PRIMARY KEY,
   user_name TEXT NOT NULL,
-  access_token INTEGER DEFAULT NULL,
-  profile_tag varchar(32) NOT NULL,
-  kind varchar(8) NOT NULL,
-  app_id varchar(64) NOT NULL,
-  app_display_name varchar(64) NOT NULL,
-  device_display_name varchar(128) NOT NULL,
-  pushkey blob NOT NULL,
+  access_token BIGINT DEFAULT NULL,
+  profile_tag VARCHAR(32) NOT NULL,
+  kind VARCHAR(8) NOT NULL,
+  app_id VARCHAR(64) NOT NULL,
+  app_display_name VARCHAR(64) NOT NULL,
+  device_display_name VARCHAR(128) NOT NULL,
+  pushkey bytea NOT NULL,
   ts BIGINT NOT NULL,
-  lang varchar(8),
-  data blob,
+  lang VARCHAR(8),
+  data bytea,
   last_token TEXT,
   last_success BIGINT,
   failing_since BIGINT,
-  FOREIGN KEY(user_name) REFERENCES users(name),
-  UNIQUE (app_id, pushkey, user_name)
+  UNIQUE (app_id, pushkey)
 );
 INSERT INTO pushers2 (id, user_name, profile_tag, kind, app_id, app_display_name, device_display_name, pushkey, ts, lang, data, last_token, last_success, failing_since)
   SELECT id, user_name, profile_tag, kind, app_id, app_display_name, device_display_name, pushkey, ts, lang, data, last_token, last_success, failing_since FROM pushers;