fix: refer to portnumber property correctly

[lowlydba]

Description

Make sure we're comparing a property that is actually returned from the listener getter

How Has This Been Tested?

We don't have AG tests due to the complexity at this time.

Types of changes

• Bug fix (non-breaking change which fixes an issue) - Fixes [Bug] ag_listener always calling Set-DbaAgListener if listener exists #206
• New feature (non-breaking change which adds functionality)

Checklist:

• I have read/followed the CONTRIBUTING document.
• I have read/followed the PR Quick Start Guide
• I have added tests to cover my changes or they are N/A.
• I have added a changelog fragment describing the changes.
• New module options/parameters include a version_added property.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (787f403) 93.68% compared to head (71631d4) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #209      +/-   ##
===========================================
+ Coverage   93.68%   100.00%   +6.31%     
===========================================
  Files          64        33      -31     
  Lines        2202       106    -2096     
===========================================
- Hits         2063       106    -1957     
+ Misses        139         0     -139     

Flag	Coverage Δ
sanity	100.00% <ø> (ø)

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see 31 files with indirect coverage changes

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[github-actions]

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Thank you for contribution!✨

This PR has been merged and the docs are now incorporated into main:
https://lowlydba.github.io/lowlydba.sqlserver/branch/main