Fix: os6: hide snmp-server community strings when remove_secret is set

[Jurgen1994]

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

With remove_secret: true, the os6 model (Dell EMC Networking OS6 / N-Series, e.g. N1548) only masks password <hash> lines in its secret filter. SNMP community strings pass through unmodified, so a line like

  snmp-server community "public" rw

is committed to the output repo in plaintext — and an rw community is effectively a write credential for the switch.

This adds one substitution to the existing cmd :secret block:

  cfg.gsub! /^(snmp-server community )\S+(.*)/, '\1<secret hidden>\2'

matching how powerconnect and procurve already treat snmp-server community lines.

Tested against a Dell N1548 running config:

  snmp-server community "public" rw
  → snmp-server community <secret hidden> rw

Non-secret lines are unaffected, and behavior is unchanged when remove_secret is not set (the :secret block doesn't run then).