Update to Rust 1.59 and 2021 edition (#3038)

## Proposed Changes

Lots of lint updates related to `flat_map`, `unwrap_or_else` and string patterns. I did a little more creative refactoring in the op pool, but otherwise followed Clippy's suggestions.

## Additional Info

We need this PR to unblock CI.

lcli/src/etl/block_efficiency.rs

@@ -74,9 +74,9 @@ async fn get_block_attestations_set<'a, T: EthSpec>(
             .graffiti()
             .as_utf8_lossy()
             // Remove commas and apostropes from graffiti to ensure correct CSV format.
-            .replace(",", "")
-            .replace("\"", "")
-            .replace("'", ""),
+            .replace(',', "")
+            .replace('"', "")
+            .replace('\'', ""),
     };
 
     let attestations = block.message().body().attestations();