Update deprecated rspamd config option

This gets rid of the following error message:
lua; antivirus.lua:109: CLAM_VIRUS [clamav]: Using attachments_only is deprecated. Please use scan_mime_parts = true instead

As per the rspamd documentation https://rspamd.com/doc/modules/antivirus.html
attachments_only = true; # Before 1.8.1
scan_mime_parts = true; # After 1.8.1

The currently used version is rspamd 3.1.

core/rspamd/conf/antivirus.conf

@@ -1,6 +1,6 @@
 {% if ANTIVIRUS == 'clamav' %}
 clamav {
-  attachments_only = true;
+  scan_mime_parts = true;
   symbol = "CLAM_VIRUS";
   type = "clamav";
   servers = "{{ ANTIVIRUS_ADDRESS }}";