/etc/apparmor.d/usr.lib.dovecot.dovecot-auth is in apparmor-profiles 2.10.95-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#
#    Copyright (C) 2009-2013 Canonical Ltd.
#    Copyright (C) 2013 Christian Boltz
#
#    This program is free software; you can redistribute it and/or
#    modify it under the terms of version 2 of the GNU General Public
#    License published by the Free Software Foundation.
#
# ------------------------------------------------------------------
# vim: ft=apparmor
#include <tunables/global>
/usr/lib/dovecot/dovecot-auth flags=(complain) {
  #include <abstractions/authentication>
  #include <abstractions/base>
  #include <abstractions/nameservice>
  #include <abstractions/wutmp>
  #include <abstractions/dovecot-common>
  capability chown,
  capability dac_override,
  @{PROC}/@{pid}/mounts r,
  /usr/lib/dovecot/dovecot-auth mr,
  /{,var/}run/dovecot/** rw,
  # required for postfix+dovecot integration
  /var/spool/postfix/private/dovecot-auth w,
  # Site-specific additions and overrides. See local/README for details.
  #include <local/usr.lib.dovecot.dovecot-auth>
}
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