Undocumented removal of nnx.Dict in flax 0.11

[jjyyxx]

It is not difficult to migrate, but it may be better to mention in the release note and the migration note.

[8bitmp3]

@IvyZX @cgarciae

[vfdev-5]

It will be reintroduced in this PR: #4897

[cgarciae]

@jjyyxx sorry for the inconvenience, we meant to delete it long ago and managed to do it in 0.11 but we've reintroduced it in #4897 as it now relevant when using Modules as Pytrees.