Run tests on Go 1.21 (#408)

* Run tests on Go 1.21

For the most part, there are no breaking changes.

However, the expired certificate is now showing "expired certificate"
although previously it showed a simpler "bad certificate" which was
hard-coded into the TLS settings test scenario.

* Simplify condition for certificate error

Instead of two `expired certificate` and `bad certificate` comparisons, we can just check for `certificate` in error output. This satisfies us when checking there is something wrong with the certificate.

Co-authored-by: Scott Blum <dragonsinth@gmail.com>

---------

Co-authored-by: Scott Blum <dragonsinth@gmail.com>

tls_settings_test.go

@@ -213,7 +213,7 @@ func TestBrokenTLS_ClientHasExpiredCert(t *testing.T) {
 		e.Close()
 		t.Fatal("expecting TLS failure setting up server and client")
 	}
-	if !strings.Contains(err.Error(), "bad certificate") {
+	if !strings.Contains(err.Error(), "certificate") {
 		t.Fatalf("expecting TLS certificate error, got: %v", err)
 	}
 }