feat: Add support for older versions of PostgreSQL

[wjhdec]

Test Version： Postgresql 12

Before retrieving index information, first check if the pg_index table has the indnullsnotdistinct column:

• If the column exists, read its value and write it to nulls_not_distinct;
• If the column does not exist, handle it using PostgreSQL's default value of false.

[stephenafamo]

Nice!

Final request, add a note to CHANGELOG.md with your changes, ending with @thanks @wjhdec