Fix on-demand SLRU download on standby starting at WAL segment boundary #8031
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If a standby is started right after switching to a new WAL segment, the request in the SLRU download request would point to the beginning of the segment (e.g. 0/5000000), while the not-modified-since LSN would point to just after the page header (e.g. 0/5000028). It's effectively the same position, as there cannot be any WAL records in between, but the pageserver rightly errors out on any request where the request LSN < not-modified since LSN.
To fix, round down the not-modified since LSN to the beginning of the page like the request LSN.
Fixes issue #8030