Replace PostmasterRandom() with pg_strong_random().

Replace PostmasterRandom() with pg_strong_random().

Our PostmasterRandmon() was imported from PostgreSQL long time ago (in
2016). In the same year PostgreSQL replaced PostmasterRandmon() with
pg_strong_random()(src/port/pg_strong_random.c). This commit follows
it.

pg_strong_random() looks better than PostmasterRandmon(), since it's
more secure and portable. Moreover no initialization is necessary.

Reviewed-by: Martijn van Duren <pgpool@list.imperialat.at>
Discussion: [pgpool-hackers: 4588] Shuffle random functions and use better random numbers
https://www.pgpool.net/pipermail/pgpool-hackers/2025-May/004589.html