PG18: use data-checksums by default in upgrades (#8230)

Checksums are now on by default in PG18: 04bec894a

Upgrade to PG18 fails with the following error:
`old cluster does not use data checksums but the new one does`

To overcome this error, we add --data-checksums option such that
clusters with PG less than 18 also use data checksums.

Fixes #8229

src/test/regress/citus_tests/common.py

@@ -137,6 +137,7 @@ def initialize_db_for_cluster(pg_path, rel_data_path, settings, node_names):
             # --allow-group-access is used to ensure we set permissions on
             # private keys correctly
             "--allow-group-access",
+            "--data-checksums",
             "--encoding",
             "UTF8",
             "--locale",