Housekeeping - Update usings and namespaces

[ChrisPulman]

What kind of change does this PR introduce?

Housekeeping

What is the current behavior?

unused usings exist
namespaces for maui using base ReactiveUI namespace

What is the new behavior?

unused usings removed
namespaces for ReactiveUI.Maui updated
API packages updated and test amended as necessary - NO API actually changed, pure formatting update.

What might this PR break?

XAML includes using the ReactiveUI namespace will need to update to ReactiveUI.Maui to standardise across all controls.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (bfc9a04) 58.37% compared to head (2b6b0e4) 59.00%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/ReactiveUI/Mixins/DependencyResolverMixins.cs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3702      +/-   ##
==========================================
+ Coverage   58.37%   59.00%   +0.62%     
==========================================
  Files         160      160              
  Lines        5776     5776              
  Branches     1029     1029              
==========================================
+ Hits         3372     3408      +36     
+ Misses       2015     1979      -36     
  Partials      389      389              

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[github-actions]

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