Revert "Spec: Mark added_snapshot_id as optional (#8600)" (#8726)

This reverts commit f7a7eb2.

format/spec.md

@@ -541,7 +541,7 @@ Manifest list files store `manifest_file`, a struct with the following fields:
 |            | _required_ | **`517 content`**              | `int` with meaning: `0: data`, `1: deletes` | The type of files tracked by the manifest, either data or delete files; 0 for all v1 manifests |
 |            | _required_ | **`515 sequence_number`**      | `long`                                      | The sequence number when the manifest was added to the table; use 0 when reading v1 manifest lists |
 |            | _required_ | **`516 min_sequence_number`**  | `long`                                      | The minimum data sequence number of all live data or delete files in the manifest; use 0 when reading v1 manifest lists |
-| _optional_ | _required_ | **`503 added_snapshot_id`**    | `long`                                      | ID of the snapshot where the  manifest file was added |
+| _required_ | _required_ | **`503 added_snapshot_id`**    | `long`                                      | ID of the snapshot where the  manifest file was added |
 | _optional_ | _required_ | **`504 added_files_count`**    | `int`                                       | Number of entries in the manifest that have status `ADDED` (1), when `null` this is assumed to be non-zero |
 | _optional_ | _required_ | **`505 existing_files_count`** | `int`                                       | Number of entries in the manifest that have status `EXISTING` (0), when `null` this is assumed to be non-zero |
 | _optional_ | _required_ | **`506 deleted_files_count`**  | `int`                                       | Number of entries in the manifest that have status `DELETED` (2), when `null` this is assumed to be non-zero |