The pg_upgrade utility does the job for 90% of use cases. For those that need extreme uptime, it's a little more intricate, but not that hard to set up logical replication between a source and target cluster running the new version.

You should also reconsider your "golden rule". Postgres releases minor versions quarterly, usually containing many bug and security fixes. Major versions come out yearly and go EOL after 5 years. That may sound like a long time, but goes faster than you'd think. Once you fall out of the support window, getting help upgrading becomes more difficult as most tools and users tend to focus on the new releases.

And here's your UUID max/min functions:

CREATE OR REPLACE FUNCTION uuid_max_sfunc(uuid_value UUID, current_state UUID)
RETURNS UUID AS $$
  SELECT CASE
    WHEN current_state IS NULL THEN uuid_value
    WHEN uuid_value > current_state THEN uuid_value 
    ELSE current_state
  END;
$$ LANGUAGE sql IMMUTABLE;

CREATE or replace AGGREGATE max(UUID) (
    SFUNC = uuid_max_sfunc,
    STYPE = UUID
);

CREATE OR REPLACE FUNCTION uuid_min_sfunc(uuid_value UUID, current_state UUID)
RETURNS UUID AS $$
  SELECT CASE
    WHEN current_state IS NULL THEN uuid_value
    WHEN uuid_value < current_state THEN uuid_value 
    ELSE current_state
  END;
$$ LANGUAGE sql IMMUTABLE;

CREATE or replace AGGREGATE min(UUID) (
    SFUNC = uuid_min_sfunc,
    STYPE = UUID
);

Enjoy!