feat: implement centralized channel mapping management

Adds OnChannelMappingDeleted method to BridgeManager for centralized
cleanup of channel mappings across all bridge types. Updates slash
commands to use centralized management and fixes method naming
inconsistencies.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

plugin.json

@@ -6,7 +6,7 @@
   "support_url": "https://github.com/mattermost/mattermost-plugin-bridge-xmpp/issues",
   "icon_path": "assets/logo.png",
   "version": "",
-  "min_server_version": "6.2.1",
+  "min_server_version": "9.5.0",
   "server": {
     "executables": {
       "darwin-amd64": "server/dist/plugin-darwin-amd64",
@@ -82,4 +82,4 @@
       "version": "v0.1.1"
     }
   }
-}
+}