Update to upstream 2.3.18

Noah Meyerhansrequested to merge
noahm/dovecot:master into master

No packaging changes beyond refreshing or dropping patches.

Drop the following patches, which are included in this upstream release:

  • mail-cache-bigendian-1.patch
  • login-proxy-Fix-potential-memory-leak-if-backend-login-fa.patch
  • plugins-imapsieve-imap-sieve-storage-Make-sure-mail_set_u.patch

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