chore(core)!: s/tx_graph::Update/TxUpdate/

chore(core)!: s/tx_graph::Update/TxUpdate/

We shouldn't refer to `tx_graph` in `bdk_core`. TxGraph happens to
consume `Update` but it doesn't conceptually own the definition of an
update to transactions.

I tried to also remove a lot of references to "graph" where they
shouldn't be. "graph" is a very general kind of data structure so we
should avoid referring to it where it's not relevant. `tx_update` is
much better than `graph_update`.