EventListener: Use [JsonSourceGenerationOptions] & cleanup

[ScarletKuro]

Description

Now when .net7 is dropped we can use the [JsonSourceGenerationOptions] API and not create JsonSerializerOptions instance on each OnEventOccur call. Moving it as static was not an option since it would introduce a regression: #8979

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or improvement to the website or code docs)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.80%. Comparing base (28bc599) to head (6ff96a4).
Report is 545 commits behind head on dev.

Files with missing lines	Patch %	Lines
...azor/Services/EventManager/EventListenerFactory.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9983      +/-   ##
==========================================
+ Coverage   89.82%   90.80%   +0.97%     
==========================================
  Files         412      407       -5     
  Lines       11878    12600     +722     
  Branches     2364     2453      +89     
==========================================
+ Hits        10670    11442     +772     
+ Misses        681      597      -84     
- Partials      527      561      +34     

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