postgresql 12.3 convert existing DB to use case-insensitive collation?
First, a warning: there used to be problems with collations on Alpine Linux, so consider using something else.
The only way to change a a database to use a case insensitive collation throughout is to create such a collation and then use
ALTER TABLE ... ALTER ... TYPE text COLLATE your_coll;

for all string columns in the database.
ICU collations cannot (yet) be used as the default collation for a database.