Fix accidental revert of import batch size

Fix accidental revert of import batch size

Overview
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We increased batch size to 50 in 5.52 but appear to have accidentally
re-set it to 5. This was presumably due to code being merged that
patched the code with the older value & hadn't picked up the new

Before
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Each batch is 5 records

After
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Each batch is 50 records

Technical Details
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There seems to be consensus this should be at least 50.
100 is also on the table but I went with Andy's comment
https://github.com/civicrm/civicrm-core/pull/24151#issuecomment-1284498891

The issue Andy points out about the 3 second delay between batches seems excessive
and worth digging into. There used to be a 30 second delay in the UI just to
let people really luxuriate in watching our batch screen - not sure
if that is still there?