BUG #19004: Incorrect lowercasing of word-final Greek capital Sigma (Σ)

The following bug has been logged on the website:

Bug reference: 19004
Logged by: Bosheng Peng
Email address: 221250180(at)smail(dot)nju(dot)edu(dot)cn
PostgreSQL version: 17.5
Operating system: windows with docker
Description:

When applying UPPER() to the Greek word 'κόσμος', PostgreSQL returns
'ΚΌΣΜΟΣ'. However, applying LOWER() to that result returns 'κόσμοσ' instead
of the original 'κόσμος'.

According to wikipedia (https://en.wikipedia.org/wiki/Sigma) there are two
forms of lowercase for Greek capital Sigma (Σ):
- If Σ is at the end of a word, it should lowercase to ς
- If Σ is in the middle of a word, it should lowercase to σ

How to repeat:
```sql
SELECT LOWER(UPPER('κόσμος'));
-- Expected: 'κόσμος'
-- Actual: 'κόσμοσ'
```