BUG #16600: Postgres can't upper lower diacritic character of Croatina alfabet

The following bug has been logged on the website:

Bug reference: 16600
Logged by: Dusan Djuric
Email address: dusan(dot)djuric(at)istratech(dot)hr
PostgreSQL version: 12.3
Operating system: Win 10 Pro (sys&input locale hr;Croatian cp:852)
Description:

Postgres can't upper lower diacritic character of Croatina alfabet
š,đ,č,ć,ž,Š,Đ,Č,Ć,Ž,

Test case:
SHOW SERVER_ENCODING; --UTF8
SHOW CLIENT_ENCODING; --UNICODE

SELECT upper('a') = upper('a') -- true
SELECT upper('A') = upper('a') -- true
SELECT upper('a') = upper('A') -- true
SELECT upper('A') = upper('A') -- true
SELECT upper('a') = 'A' --

SELECT upper('š') = upper('š') -- true
SELECT upper('Š') = upper('š') -- false
SELECT upper('š') = upper('Š') -- false
SELECT upper('Š') = upper('Š') -- true
SELECT upper('š') = 'Š' -- false

SELECT
'šđč枊ĐČĆŽ',upper('šđč枊ĐČĆŽ'),lower('šđč枊ĐČĆŽ')--"šđč枊ĐČĆŽ" "šđč枊ĐČĆŽ" "šđč枊ĐČĆŽ"