Fix Bayes spamfilter issue

fix GH-4401
The string "Bayes" includes the string "yes" so the previous "contains" approach would mark everything from Bayes as spam.

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Co-authored-by: divinity76 <hans@loltek.net>

install/deb/exim/system.filter

@@ -1,4 +1,4 @@
-if $h_X-Spam-Status: contains "Yes"
+if $h_X-Spam-Status: matches ^[Yy]es
 then
     headers add "Old-Subject: $h_subject"
     headers remove "Subject"