[main] Upgrade Rust toolchain (#6206)

This is our monthly toolchain upgrade to the latest stable version. Since iotedge/edgelet is a binary package, we want to be tracking Rust compiler releases closely in the event a stdlib vulnerability is found. We do not pin to "stable", however, since that has caused pipeline breakage when some code patterns are made illegal (like in the upgrade from 1.47 to 1.48 [^0]) or, more often, clippy lints are added.

- Add `#[must_use]` directives on methods returning `Self`
- Change `unwrap_or_else(...::new)` to `unwrap_or_default` where appropriate
- Box `CertIssuanceOptions` in `EdgeCa` enum due to large variant size difference otherwise

## Azure IoT Edge PR checklist:

edgelet/edgelet-core/src/certificate_properties.rs

@@ -43,6 +43,7 @@ impl CertificateProperties {
         &self.common_name
     }
 
+    #[must_use]
     pub fn with_common_name(mut self, common_name: String) -> Self {
         self.common_name = common_name;
         self
@@ -52,6 +53,7 @@ impl CertificateProperties {
         &self.certificate_type
     }
 
+    #[must_use]
     pub fn with_certificate_type(mut self, certificate_type: CertificateType) -> Self {
         self.certificate_type = certificate_type;
         self
@@ -61,6 +63,7 @@ impl CertificateProperties {
         &self.alias
     }
 
+    #[must_use]
     pub fn with_alias(mut self, alias: String) -> Self {
         self.alias = alias;
         self
@@ -70,6 +73,7 @@ impl CertificateProperties {
         &self.issuer
     }
 
+    #[must_use]
     pub fn with_issuer(mut self, issuer: CertificateIssuer) -> Self {
         self.issuer = issuer;
         self
@@ -79,6 +83,7 @@ impl CertificateProperties {
         self.dns_san_entries.as_ref().map(AsRef::as_ref)
     }
 
+    #[must_use]
     pub fn with_dns_san_entries(mut self, entries: Vec<String>) -> Self {
         self.dns_san_entries = Some(entries);
         self
@@ -88,6 +93,7 @@ impl CertificateProperties {
         self.ip_entries.as_ref().map(AsRef::as_ref)
     }
 
+    #[must_use]
     pub fn with_ip_entries(mut self, entries: Vec<String>) -> Self {
         self.ip_entries = Some(entries);
         self

edgelet/edgelet-core/src/identity.rs

@@ -52,6 +52,7 @@ impl IdentitySpec {
         self.generation_id.as_ref().map(AsRef::as_ref)
     }
 
+    #[must_use]
     pub fn with_generation_id(mut self, generation_id: String) -> Self {
         self.generation_id = Some(generation_id);
         self
@@ -61,6 +62,7 @@ impl IdentitySpec {
         self.managed_by.as_ref().map(AsRef::as_ref)
     }
 
+    #[must_use]
     pub fn with_managed_by(mut self, managed_by: String) -> Self {
         self.managed_by = Some(managed_by);
         self