Immutable functions in GENERATED columns fail

[AbelVM]

Using GENERATED columns using h3-pg functions fail with this message, but the functions are actually IMMUTABLE (as well as the wrapped ones):

ERROR: generation expression is not immutable

Examples:

CREATE TABLE generatetemp (  
	h3_id h3index ,
        geom geometry(Point, 4326) GENERATED ALWAYS AS (h3_to_geometry(h3_id)) STORED	
) ;	

Function: https://github.com/bytesandbrains/h3-pg/blob/master/sql/install/99-postgis.sql#L30

CREATE TABLE generatetemp (  
	geom geometry(Point, 4326),
	h3_id h3index GENERATED ALWAYS AS (h3_geo_to_h3(geom, 10)) STORED	         
) ;	

Function: https://github.com/bytesandbrains/h3-pg/blob/master/sql/install/99-postgis.sql#L22

[zachasme]

This is probably because we forgot to include an upgrade-path in bd526aa.

We should mark IMMUTABLE STRICT PARALLEL SAFE in an upgrade-file.