Incorrect lowercasing of word-final Greek capital Sigma (Σ)

When applying UPPER() to the Greek word 'κόσμος', MariaDB 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'));
```