[deps] revert the recent package:markdown rev (b/255935594)
[deps] revert the recent package:markdown rev (b/255935594)

Change-Id: I503e2b1c5d09c1e130cc105e40500b73d3bed7d9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266240
Reviewed-by: Oleh Prypin <5baa3e04f9bd6ba7a2a4b56625a3c84f19237cdb@google.com>
Auto-Submit: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Commit-Queue: Oleh Prypin <5baa3e04f9bd6ba7a2a4b56625a3c84f19237cdb@google.com>
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk