Merge upstream v0.16.23

Five conflicts, resolved:

- crates/common/src/auth/authentication.rs: upstream's get_directory_for_token
  and JwtClaims replace extract_jwt_domain; the per-domain directory code
  (DIR-1, DIR-5 to DIR-7) is kept, and the token lookup routes through it.
  The release's one new Enterprise snippet was the body of
  get_directory_for_issuer, which stays returning None: a token naming no
  address gets the server default, as DIR-2 specifies and as v0.16.22 did.
- crates/common/src/manager/application.rs: upstream's rewrite of the tests,
  with the temp directory names renamed again, and the 5(a) notice the
  name-purge change should have added.
- crates/common/src/network/mta.rs: both sides' imports.
- crates/main/Cargo.toml: the AGPL-only license kept, version 0.16.23.
- Cargo.lock: upstream's, with the fork's crates added by Cargo.

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"),