Bump stalwartlabs/mail-server from v0.7.3 to v0.8.4

Bumps stalwartlabs/mail-server from v0.7.3 to v0.8.4.

---
updated-dependencies:
- dependency-name: stalwartlabs/mail-server
  dependency-type: direct:production
...

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

Dockerfile

@@ -1,5 +1,5 @@
 # From https://github.com/stalwartlabs/mail-server/blob/main/Dockerfile
-FROM stalwartlabs/mail-server:v0.7.3
+FROM stalwartlabs/mail-server:v0.8.4
 
 COPY --chmod=664 config.toml /opt/stalwart-mail/etc/config.toml
 COPY --chmod=775 start.sh /start.sh