BUG #15805: Problem with lower function for greek sigma (Σ) letter

The following bug has been logged on the website:

Bug reference:      15805
Logged by:          Sergey kuznetsov
Email address:      iksss.88@gmail.com
PostgreSQL version: 9.6.10
Operating system:   linux
Description:

I see unexpected behaviour of lower function for greek sigma letter Σ.
According to wikipedia (https://en.wikipedia.org/wiki/Sigma) if this letter
is in final word position, it should be ς in lowercase, and not σ. But
PotgreSQL lower function returns σ, for example lower('ΔΗΜΟΤΕΣ ΦΑΙΣΤΟΥ') =
"δημοτεσ φαιστου" instead of "δημοτες φαιστου". So if I try to pass this
lower string from another part of a system (java code, for example) Postgre
will not return this row cause it differs from java-generated one.

Thanks, 
Sergey Kuznetsov