housekeeping: Upgraded Xamarin.Forms to v4.5

[RLittlesII]

What kind of change does this PR introduce?

General housekeeping. Xamarin.Forms 4.6 is coming out in the next cycle based on their current release cadence, moving this change in now because of consumer reported issues.

Resolves: #2386

What is the current behavior?

Xamarin 4.4.* is the minimum required version.

What is the new behavior?

Xamarin 4.5.* is the minimum required version.

What might this PR break?

ReactiveUI.XamForms

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 #2388 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2388   +/-   ##
=======================================
  Coverage   54.66%   54.66%           
=======================================
  Files         114      114           
  Lines        4341     4341           
  Branches      663      663           
=======================================
  Hits         2373     2373           
  Misses       1801     1801           
  Partials      167      167           

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[github-actions]

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