dovecot: fix deprecated keys in plist

The "OnDemand" and "ServiceDescription" were deprecated in OS X 10.5.
Replace "OnDemand" with "KeepAlive" and delete the "ServiceDescription" key.

Closes Homebrew/legacy-homebrew#43775.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/dovecot.rb

@@ -45,7 +45,7 @@ def plist; <<-EOS.undent
       <dict>
         <key>Label</key>
         <string>#{plist_name}</string>
-        <key>OnDemand</key>
+        <key>KeepAlive</key>
         <false/>
         <key>RunAtLoad</key>
         <true/>
@@ -54,8 +54,6 @@ def plist; <<-EOS.undent
           <string>#{opt_sbin}/dovecot</string>
           <string>-F</string>
         </array>
-        <key>ServiceDescription</key>
-        <string>Dovecot mail server</string>
         <key>StandardErrorPath</key>
         <string>#{var}/log/dovecot/dovecot.log</string>
         <key>StandardOutPath</key>