Remove references to old function name

Remove references to old function name

In a97bbe1f1df I accidentally referenced heapgetpage(), both in a function
name and a comment. But since 44086b09753 the relevant function is named
heap_prepare_pagescan().  Rename the new function to page_collect_tuples().

Reported-by: Melanie Plageman <melanieplageman@gmail.com>
Reported-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/20240407172615.cocrsvboqm3ttqe4@awork3.anarazel.de
Discussion: https://postgr.es/m/CAApHDvp4SniHopTrVeKWcEvNXFtdki0utAvO=5R7H6TNhtULRQ@mail.gmail.com