[Apprise] Add deprecation warning about outdated usage

Author: amotl

HANDBOOK.md

@@ -600,7 +600,7 @@ baseuri  = 'json://localhost:1234/mqtthook'
 ; https://github.com/caronc/apprise/wiki/Notify_discord
 ; https://discord.com/developers/docs/resources/webhook
 ; discord://{WebhookID}/{WebhookToken}/
-module   = 'apprise'
+module   = 'apprise_single'
 baseuri  = 'discord://4174216298/JHMHI8qBe7bk2ZwO5U711o3dV_js'
 
 [config:apprise-ntfy]

mqttwarn/services/apprise.py

@@ -1 +1,7 @@
+import warnings
+
 from mqttwarn.services.apprise_single import plugin
+
+warnings.warn("`mqttwarn.services.apprise` will be removed in a future release of mqttwarn. "
+              "Please use `mqttwarn.services.apprise_single` or `mqttwarn.services.apprise_multi` instead.",
+              category=DeprecationWarning)