Fix Broken Snapshots in Mixed Clusters (elastic#48993) (elastic#48995)

Reverts elastic#48947 and fixes the issue orginally addressed by removing the assertion.
It turns out we can't simply pass empty shard generations to the snapshot finalization in the
BwC case as that results in no indices being added to the meta for the given snapshot since
we take the indices from the shard generations (even in the BwC case the `null` generations work
fine for this).

Closes elastic#48983

server/src/main/java/org/elasticsearch/repositories/RepositoryData.java

@@ -315,9 +315,6 @@ public List<IndexId> resolveNewIndices(final List<String> indicesToResolve) {
      * Writes the snapshots metadata and the related indices metadata to x-content.
      */
     public XContentBuilder snapshotsToXContent(final XContentBuilder builder, final boolean shouldWriteShardGens) throws IOException {
-        assert shouldWriteShardGens || shardGenerations.indices().isEmpty() :
-            "Should not build shard generations in BwC mode but saw generations [" + shardGenerations + "]";
-
         builder.startObject();
         // write the snapshots list
         builder.startArray(SNAPSHOTS);

server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java

@@ -594,9 +594,6 @@ private void cleanupAfterError(Exception exception) {
     }
 
     private static ShardGenerations buildGenerations(SnapshotsInProgress.Entry snapshot) {
-        if (snapshot.useShardGenerations() == false) {
-            return ShardGenerations.EMPTY;
-        }
         ShardGenerations.Builder builder = ShardGenerations.builder();
         final Map<String, IndexId> indexLookup = new HashMap<>();
         snapshot.indices().forEach(idx -> indexLookup.put(idx.getName(), idx));