ResizeObserver: Clean up, make implementations internal, add xmodoc

[ScarletKuro]

Description

Improves overall code quality
The implementation should always be internal.
Removed IDiposable.

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 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.17%. Comparing base (6ac51c8) to head (d9b53b3).
Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
...r/Services/ResizeObserver/ResizeObserverFactory.cs	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10055      +/-   ##
==========================================
+ Coverage   91.10%   91.17%   +0.07%     
==========================================
  Files         410      410              
  Lines       12476    12471       -5     
  Branches     2421     2420       -1     
==========================================
+ Hits        11366    11371       +5     
+ Misses        565      555      -10     
  Partials      545      545              

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[jperson2000]

Looking good! Just had one XML doc suggestion

[ScarletKuro]

Added to the v8 migration guide at #9953