The following are the initial fixes for PG15:

1) From PG 15 onward, the datlastsysoid has been removed from the table pg_database.
    We have added the constant _DATABASE_LAST_SYSTEM_OID = 16383, all the objects below
    this value are considered to be system objects. Modified the pgAdmin logic accordingly.

 2) Concatenation operator '||' needs a specific typecast to be applied to query variables. Modified SQL's accordingly.

Fixes #7283

web/pgadmin/utils/driver/psycopg2/connection.py

@@ -526,7 +526,7 @@ class Connection(BaseConnection):
 SELECT
     db.oid as did, db.datname, db.datallowconn,
     pg_encoding_to_char(db.encoding) AS serverencoding,
-    has_database_privilege(db.oid, 'CREATE') as cancreate, datlastsysoid,
+    has_database_privilege(db.oid, 'CREATE') as cancreate,
     datistemplate
 FROM
     pg_catalog.pg_database db

web/pgadmin/utils/driver/psycopg2/server_manager.py

@@ -217,7 +217,7 @@ SELECT
     db.oid as did, db.datname, db.datallowconn,
     pg_catalog.pg_encoding_to_char(db.encoding) AS serverencoding,
     pg_catalog.has_database_privilege(db.oid, 'CREATE') as cancreate,
-    datlastsysoid, datistemplate
+    datistemplate
 FROM
     pg_catalog.pg_database db
 WHERE db.oid = {0}""".format(did))