[symfony/contracts] Why hard dependency to php 7.2.9?

**Symfony version(s) affected**: v3.4.35

**Description**  
I can't install symfony/contracts on a SUSE SLES 12 (SP4) system since it only has php 7.2.**5**. symfony/contracts requires 7.2.**9**.

**Possible Solution**  
Set the dependency to `^7.2` insteadt of `^7.2.9`.

**Additional context**  
If a pull reuqest is needed, just tell me.
