BUG #16565: Systemd service file does not create /var/run/postgresql directory across reboot

The following bug has been logged on the website:

Bug reference:      16565
Logged by:          Marco Cuccato
Email address:      mcuccato.vts@gmail.com
PostgreSQL version: 12.3
Operating system:   Red Hat Enterprise Linux Server release 7.6
Description:

When the server is rebooted, PostgreSQL service doesn't start because of
this error:
FATAL:  could not create lock file "/var/run/postgresql/.s.PGSQL.5432.lock":
No such file or directory

This happens because it tries to create the lock file under
/var/run/postgresql that doesn't exists. It doesn't exist because /var/run
is a symlink to ../run (/run as absolute path) that is mounted as tmpfs.
As tmpfs is not persistent, when the server is rebooted the directory
doesn't exists and must be re-created.
Doing that, the lock file will be created successfully and the service will
start up correctly.

Please fix the postgresql-12.service systemd file: make it check and
eventually create /var/run/postgresql directory.
Thanks