go-dep: bump github.com/pocketbase/pocketbase in /backend

Bumps [github.com/pocketbase/pocketbase](https://github.com/pocketbase/pocketbase) from 0.18.3 to 0.18.4.
- [Release notes](https://github.com/pocketbase/pocketbase/releases)
- [Changelog](https://github.com/pocketbase/pocketbase/blob/master/CHANGELOG.md)
- [Commits](pocketbase/pocketbase@v0.18.3...v0.18.4)

---
updated-dependencies:
- dependency-name: github.com/pocketbase/pocketbase
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/labstack/echo/v5 v5.0.0-20230722203903-ec5b858dab61
 	github.com/mdlayher/wol v0.0.0-20220221231636-b763a792253a
 	github.com/pocketbase/dbx v1.10.1
-	github.com/pocketbase/pocketbase v0.18.3
+	github.com/pocketbase/pocketbase v0.18.4
 	github.com/prometheus-community/pro-bing v0.3.0
 	github.com/robfig/cron/v3 v3.0.1
 )