Bump ssz_types to v0.12.2 (#8032)

https://github.com/sigp/lighthouse/issues/8012


  Replace all instances of `VariableList::from` and `FixedVector::from` to their `try_from` variants.

While I tried to use proper error handling in most cases, there were certain situations where adding an `expect` for situations where `try_from` can trivially never fail avoided adding a lot of extra complexity.


Co-Authored-By: Mac L <mjladson@pm.me>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

testing/ef_tests/src/cases/ssz_generic.rs

@@ -318,14 +318,13 @@ where
 {
     let s: String = serde::de::Deserialize::deserialize(deserializer)?;
     let decoded: Vec<u8> = hex::decode(&s.as_str()[2..]).map_err(D::Error::custom)?;
+    let decoded_len = decoded.len();
 
-    if decoded.len() > N::to_usize() {
-        Err(D::Error::custom(format!(
+    decoded.try_into().map_err(|_| {
+        D::Error::custom(format!(
             "Too many values for list, got: {}, limit: {}",
-            decoded.len(),
+            decoded_len,
             N::to_usize()
-        )))
-    } else {
-        Ok(decoded.into())
-    }
+        ))
+    })
 }