Fixup some StringInfo usages

Similar to f736e188c, I've attached a patch that fixes up a few
misusages of the StringInfo functions. These just swap one function
call for another function that is more suited to the use case.

I've also attached the patch that I used to find these.  That's not
intended for commit.

I feel like it's a good idea to fix these soon while they're new
rather than wait N years and make backpatching bug fixes possibly
harder.

I've attached a file with git blame commands to identify where all of
these were introduced.  All are new to PG17.

Any objections?

David