PG18 - unify psql headings to ‘List of relations’ (#8119)

fixes #8110 

This patch updates the `normalize.sed` script used in pg18 psql
regression tests:

- Replaces the headings “List of tables”, “List of indexes”, and “List
of sequences” with a single, uniform heading: “List of relations”.

src/test/regress/bin/normalize.sed

@@ -339,3 +339,8 @@ s/(actual rows=[0-9]+)\.[0-9]+/\1/g
 # ignore XML <Disabled>true</Disabled> or <Disabled>false</Disabled>
 /^\s*<Disabled>.*<\/Disabled>/d
 # pg18 “Disabled” change end
+
+# PG18 psql: headings changed from "List of relations" to per-type titles
+s/^([ \t]*)List of tables$/\1List of relations/g
+s/^([ \t]*)List of indexes$/\1List of relations/g
+s/^([ \t]*)List of sequences$/\1List of relations/g