Replace cargo-deny skip-tree with explicit skips (#1468)

### What

Replace cargo-deny `skip-tree` with explicit `skip`s for each dependency
we wish to allow duplicates.

### Why

The `skip-tree` obfuscates the dependencies that we're allowing to be
duplicated, and even seems to hide some dependencies being duplicated
that don't seem to be part of the soroban-wasmi dependency graph that we
depend. This may be due to a bug, or possibly the dep is in the dev
dependencies of soroban-wasmi, or some feature that we don't use.

I became aware of this issue because I noticed that in #1456 the
Cargo.lock file gained a duplicate dependency on the `syn` crate, having
already a dependency on version `2.0.39` and gaining a dependency on
version `1.0.109`. This is fine and not a problem in and of itself – we
can't eradicate all duplicates – except that the `deny.toml` file gained
no new entry, no statement to say that we were okay with this new
duplicate.

In my own testing locally it appears that when the `skip-tree` entry
containing `soroban-wasmi` is present, the `syn` duplicate dependency is
hidden/silenced, even though it was not a dependency prior to the change
that did not involve the `soroban-wasmi` crate.

The `skip-tree` config obfuscates the duplicate dependencies in a couple
ways.

Firstly due to the odd behaviour detailed above we appear to not get
notified of new duplicate dependencies, even if they do not appear to be
an active in use dependency in our existing graph.

Secondly because the `skip-tree` communicates only partially about what
is being allowed. In our case the `skip-tree` allows two versions of the
`hashbrown` dep, but we actually need the two versions for reasons other
than `soroban-wasmi`, so it doesn't tell the full story.

Listing out the duplicate dependencies one-by-one is explicit, and it
helps us at a glance know what our dupes are, and how many we have, as a
signal to "how bad it is getting".

### Known limitations

N/A

deny.toml

@@ -234,15 +234,15 @@ deny = [
 
 # Certain crates/versions that will be skipped when doing duplicate detection.
 skip = [
- #{ name = "ansi_term", version = "=0.11.0" },
+ { name = "hashbrown", version = "=0.13.2" },
+ { name = "syn", version = "=1.0.109" },
 ]
 # Similarly to `skip` allows you to skip certain crates during duplicate
 # detection. Unlike skip, it also includes the entire tree of transitive
 # dependencies starting at the specified crate, up to a certain depth, which is
 # by default infinite.
 skip-tree = [
- { name = "soroban-wasmi" }
- ]
+]
 
 # This section is considered when running `cargo deny check sources`.
 # More documentation about the 'sources' section can be found here: