archweb: reduce mirrorlog storage duration to 6 months

One year of mirrorlogs keeps a table of 5000 MB which is rather large as
we only show 7 days of logs. Keep it 6 months as maybe in the future
mirror operators are interested in older data.

roles/archweb/templates/local_settings.py.j2

@@ -80,4 +80,7 @@ TIER0_MIRROR_DOMAIN = 'repos.archlinux.org'
 
 MASTODON_LINK = 'https://fosstodon.org/@archlinux'
 
+# Keep mirrorlogs around for 6 months
+MIRRORLOG_RETENTION_PERIOD = 180
+
 # vim: set ts=4 sw=4 et: