[SignatureBot] Add or update signature nucleitemplates_mailgun-takeover.yml

[liquidsec]

Add or update signature: nucleitemplates_mailgun-takeover.yml

This PR adds or updates the follow signature:

identifiers:
  cnames: []
  ips: []
  nameservers: []
  not_cnames: []
matcher_rule:
  matchers:
  - dsl:
    - Host != ip
    type: dsl
  - status: 404
    type: status
  - condition: and
    part: cname
    type: word
    words:
    - mailgun.org
  matchers-condition: and
mode: http
service_name: Mailgun Takeover Detection
source: nucleitemplates