add --no-sync to pg_upgrade's calls to pg_dump and pg_dumpall

This is likely small potatoes compared to some of the other
pg_upgrade-related improvements I've proposed [0] [1] or plan to propose,
but this is easy enough, and I already wrote the patch, so here it is.
AFAICT there's no reason to bother syncing these dump files to disk.  If
someone pulls the plug during pg_upgrade, it's not like you can resume
pg_upgrade from where it left off.  Also, I think we skipped syncing before
v10, anyway, as the --no-sync flag was only added in commit 96a7128, which
added the code to sync dump files, too.

[0] https://postgr.es/m/20240418041712.GA3441570%40nathanxps13
[1] https://postgr.es/m/20240503025140.GA1227404%40nathanxps13

-- 
Nathan Bossart
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