Update fortios.rb to remove additional secrets

[codejake]

"config system acme" uses "set privatekey" instead of the usual "set private-key" stanza on 7.0.x (tested on 7.0.13).

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

[codejake]

Example config snippet from FortiOS 7.0.13:

config system acme
    set interface "port30"
    set source-ip 0.0.0.0
    set source-ip6 ::
    config accounts
        edit "ACME-.letsencrypt.org-0000"
            set status "valid"
            set url "https://acme-v02.api.letsencrypt.org/acme/acct/829626287"
            set ca_url "https://acme-v02.api.letsencrypt.org/directory"
            set email "jake@nobody.com"
            set privatekey "-----BEGIN ENCRYPTED PRIVATE KEY-----
MIIHbTBXBgkqhkiG9w0BBQ0wSjApBgkqhkiG9w0BQQwwHAQI9fFXTPkTat8CAggA
MAwGCCqGSIb3DQIJBQAwHQYJYIZIAWUDBAEqBBA2B0dIE1D8JFIbwRo5a9KXBIIH