Reduce Rust MSRV to 1.56 (Qiskit#8077)

Commit 84cfd5c recently used a `&[char; 2]` as the type in the
constructor of the `std::Pattern` trait, which is a feature of Rust
unstable, and only available in newer versions of Rust.  We've not fixed
an absolute minimum supported Rust version, but have generally tried to
keep things building with at least the last six months' releases.

src/results/marginalization.rs

@@ -18,11 +18,16 @@ fn marginalize<T: std::ops::AddAssign + Copy>(
     indices: Option<Vec<usize>>,
 ) -> HashMap<String, T> {
     let mut out_counts: HashMap<String, T> = HashMap::with_capacity(counts.len());
-    let clbit_size = counts.keys().next().unwrap().replace(&['_', ' '], "").len();
+    let clbit_size = counts
+        .keys()
+        .next()
+        .unwrap()
+        .replace(|c| c == '_' || c == ' ', "")
+        .len();
     let all_indices: Vec<usize> = (0..clbit_size).collect();
     counts
         .iter()
-        .map(|(k, v)| (k.replace(&['_', ' '], ""), *v))
+        .map(|(k, v)| (k.replace(|c| c == '_' || c == ' ', ""), *v))
         .for_each(|(k, v)| match &indices {
             Some(indices) => {
                 if all_indices == *indices {