Out of sync practice exercises

[github-actions]

Out of sync practice exercises have been detected:

Command: configlet sync --tests --docs --metadata --filepaths

Output:

Checking exercises...
[warn] docs: instructions unsynced: affine-cipher
[warn] docs: instructions unsynced: flatten-array
[warn] docs: instructions unsynced: grains
[warn] docs: introduction missing: grains
[warn] docs: instructions unsynced: sieve
[warn] docs: instructions unsynced: simple-cipher
[warn] flatten-array: missing 4 test cases
       - consecutive null values at the front of the array are omitted from the final result (bc72da10-5f55-4ada-baf3-50e4da02ec8e)
       - consecutive null values in the middle of the array are omitted from the final result (6991836d-0d9b-4703-80a0-3f1f23eb5981)
       - 6 level nested array with null values (dc90a09c-5376-449c-a7b3-c2d20d540069)
       - all values in nested array are null (51f5d9af-8f7f-4fb5-a156-69e8282cb275)
[warn] some exercises have unsynced docs
[warn] some exercises are missing test cases

[BNAndras]

Closed by #458 and #459