test: regenerate L2TipsKVStore snapshot for v4-next

v4-next's L2TipsStore predates the checkpoint concept and so does not write the 'proposedCheckpoint' key. Removed that entry from the snapshot so the new compatibility test passes against the v4-next storage layout. No source code changes; only the per-store snapshot file.

Author: AztecBot

yarn-project/pxe/src/storage/backwards_compatibility_tests/__snapshots__/L2TipsKVStore.json

@@ -8,10 +8,6 @@
       "key": "utf8:proposed",
       "value": "num:179"
     },
-    {
-      "key": "utf8:proposedCheckpoint",
-      "value": "num:71"
-    },
     {
       "key": "utf8:proven",
       "value": "num:79"