Add egress to banned list of instance names

[hardillb]

part of FlowFuse/CloudProject#521

Description

This is so we can have a DNS entry for the FF Cloud egress IP address

Related Issue(s)

FlowFuse/CloudProject#521

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[hardillb]

Wondering if this list should be configurable for admins?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.05%. Comparing base (4e1f5ec) to head (77264d5).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4533   +/-   ##
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  Coverage   78.05%   78.05%           
=======================================
  Files         300      300           
  Lines       14270    14270           
  Branches     3237     3237           
=======================================
  Hits        11138    11138           
  Misses       3132     3132           

Flag	Coverage Δ
backend	78.05% <ø> (ø)

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[knolleary]

We can make it configurable if/when there's a need to do so

[hardillb]

I was more thinking this is a list that applies to all instances, but is pretty specific to FF Cloud