Looking through RSReportServer.Config description that value seems not to be accessible (pun) to configurators.
SSRS as you note is now designed for features found in version 1.7 upwards but may work primarily using 1.3 compatible objects.
The simplest solution may be to use a binary/hex patchier to replace the 8 bytes %PDF-1.7 header string with %PDF-1.3 (do NOT use an ascii string find and replace such as sed/grep), if you want to try the dirty way you need either https://sourceforge.net/projects/bbe- or other equivalent to hexdump then modify then hex2bin and verify no byte change in file size (not even one byte +/- since the trailer would be corrupted)
However there is no guarantee that any 1.5 or above objects will not break FPDI so for best compatibility you could parse through Ghostscript to "down-level"
for more details see related question and old link here https://stackoverflow.com/a/51002675/10802527
Do follow the link to iconic blog but the modern command line can be shorter so try the traditional
<?php
exec('gs -sDEVICE=pdfwrite -o="'.$srcfile_new.'" "'.$srcfile.'"');
?>
and also try for comparison using 9.55 additional -dNEWPDF
