Take advantage of changes in recent Go versions (#3361)

Given that #2714 wasn't merged but we are now at a minimum supported Go
version of 1.20 (soon to be 1.21), I wanted to carry over some of the
changes. Namely:
- Fix the log typo
- Simplify build constraints for unix
- Use stdlib atomic package

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [x] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `0x1a8510f2 <admin@0x1a8510f2.space>`

---------

Co-authored-by: devonh <devon.dmytro@gmail.com>

clientapi/routing/routing.go

@@ -255,7 +255,7 @@ func Setup(
 		logrus.Info("Enabling server notices at /_synapse/admin/v1/send_server_notice")
 		serverNotificationSender, err := getSenderDevice(context.Background(), rsAPI, userAPI, cfg)
 		if err != nil {
-			logrus.WithError(err).Fatal("unable to get account for sending sending server notices")
+			logrus.WithError(err).Fatal("unable to get account for sending server notices")
 		}
 
 		synapseAdminRouter.Handle("/admin/v1/send_server_notice/{txnID}",