Has the postgres sorting order changed between versions 11.5 and 13
The sorting order for strings in PostgreSQL depends on the collation used.
This collation is taken either from the operating system C library or from the ICU library; if you didn't specify anything, it is the C library.
So if your sorting order changed, that means that your C library changed. There is no direct connection to PostgreSQL.
Note that when you upgrade the operating system C library, you should reindex all indexes on string columns to avoid data corruption.