Import upstream v0.16.23, stripped

Upstream commit: 9d1c75ab68435e4417337f768291e5f947686203
Enterprise-only files removed or emptied: 63
Enterprise-only snippets removed: 118 in 50 files
Dangling module declarations removed: 5
Edits turning enterprise off: 25
Third-party code: 14 files, 0 not in THIRD-PARTY.md
Verification: clean

One snippet more than v0.16.22, in crates/common/src/auth/authentication.rs
(3, was 2).

crates/common/src/config/smtp/resolver.rs

@@ -214,6 +214,7 @@ impl Resolvers {
         let config_dnssec = resolver_config.clone();
         let mut opts_dnssec = opts.clone();
         opts_dnssec.validate = true;
+        opts_dnssec.num_concurrent_reqs = 1;
 
         let dnssec = DnssecResolver {
             resolver: TokioResolver::builder_with_config(
@@ -343,6 +344,7 @@ impl Default for Resolvers {
         let config_dnssec = config.clone();
         let mut opts_dnssec = opts.clone();
         opts_dnssec.validate = true;
+        opts_dnssec.num_concurrent_reqs = 1;
 
         Self {
             dns: MessageAuthenticator::new(config, opts).expect("Failed to build DNS resolver"),