smtpsrv: Add comment on BuildNameToCertificate being deprecated

tls.Config.BuildNameToCertificate was deprecated in Go 1.14, and is no
longer necessary.

However, we support down to 1.11, so we will keep it for now.

This patch adds a TODO to remove it in the future once the minimum
supported version is 1.14; and adjust the CI linter accordingly.

.cirrus.yml

@@ -53,7 +53,9 @@ lint_task:
               - errcheck
           - linters:
               - staticcheck
-            text: "SA1015:"
+            # SA1015: time.Tick leaks but it's ok on endless functions.
+            # SA1019: BuildNameToCertificate deprecation, remove on Go >= 1.14.
+            text: "SA1015:|SA1019:"
         exclude-use-default: true
   lint_config_file:
     path: /tmp/lint.yml

internal/smtpsrv/server.go

@@ -202,6 +202,8 @@ func (s *Server) ListenAndServe() {
 
 	// At this point the TLS config should be done, build the
 	// name->certificate map (used by the TLS library for SNI).
+	// TODO: Once we support only Go >= 1.14, we can drop this, as it is no
+	// longer necessary.
 	s.tlsConfig.BuildNameToCertificate()
 
 	go s.periodicallyReload()