housekeeping: Upgraded Xamarin.Forms to v4.6

[RLittlesII]

What kind of change does this PR introduce?

Upgrades Xamarin.Forms to v4.6

What is the current behavior?

Targeting Xamarin.Forms to v4.5

What is the new behavior?

Targeting Xamarin.Forms to v4.6

What might this PR break?

Nothing

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

Merging #2443 into main will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2443      +/-   ##
==========================================
+ Coverage   52.67%   52.71%   +0.04%     
==========================================
  Files         114      114              
  Lines        4450     4450              
  Branches      685      685              
==========================================
+ Hits         2344     2346       +2     
+ Misses       1925     1922       -3     
- Partials      181      182       +1     

Impacted Files	Coverage Δ
src/ReactiveUI.Winforms/ViewModelViewHost.cs	89.58% <0.00%> (+2.08%)	⬆️

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[github-actions]

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