Change prefetch logic in vacuum

[knizhnik]

See https://neondb.slack.com/archives/C03QLRH7PPD/p1745003314183649

Vacuum doesn't use prefetch because this strange logic in lazy_scan_heap:

			/* And only up to the next unskippable block */
			if (next_prefetch_block + prefetch_budget > vacrel->next_unskippable_block)
				prefetch_budget = vacrel->next_unskippable_block - next_prefetch_block;

Solution:

Disable prefetch only if vacuum jumps to next skippable block (there is SKIP_PAGES_THRESHOLD) which cancel seqscan and perform jump only if gap iOS larger enough).