Error updating from version 1.3 to version 1.4

When upgrading pg_cron from version 1.3 to version 1.4 we are seeing the following error

```
postgres=# ALTER EXTENSION pg_cron UPDATE;
ERROR: function "schedule" already exists with same argument types
```
System info:

```
postgres=# SELECT version();
version
---------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 13.5 (Ubuntu 13.5-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit
(1 row)

postgres=# select * from pg_available_extensions where name='pg_cron';
name | default_version | installed_version | comment
---------+-----------------+-------------------+------------------------------
pg_cron | 1.4 | 1.3 | Job scheduler for PostgreSQL
(1 row)
postgres=# \dx pg_cron
List of installed extensions
Name | Version | Schema | Description
---------+---------+--------+------------------------------
pg_cron | 1.3 | public | Job scheduler for PostgreSQL
(1 row)

postgres=# \dx+ pg_cron
Objects in extension "pg_cron"
Object description
----------------------------------------
function cron.job_cache_invalidate()
function cron.schedule(name,text,text)
function cron.schedule(text,text)
function cron.unschedule(bigint)
function cron.unschedule(name)
schema cron
sequence cron.jobid_seq
sequence cron.runid_seq
table cron.job
table cron.job_run_details
(10 rows)
```