sql/pg_catalog: fix attname for dropped columns

Previously, the attname columns for dropped columns was padded
used a %8d string format. This led to incorrect behavior as it meant
a total of 8 '.' characters inclusive of the column ordinal as compared to
the PG code base which always pads with 8 '.'. This affected migration tooling
since it attempts to verify column names using the returned attname and led to
mismatching column reports.

This commit fixes the issue by explicitly setting the padding to 8 '.' to match
the PG implementation.

Fixes: #120855

Release note (bug fix): attname for dropped columns is now properly padded
with 8 '.' to match PG.