Remove CA2211 NoWarn

[gabidabet]

Pull Request

The issue or feature being addressed

#1290

Details on the issue fix or feature implementation

I found that both classes SystemClock and TaskHelper are declared in the Polly/PublicAPI.Shipped.txt, so I opt for disabling the warning locally to prevent adding new cases without editing the shipped API.

We can make the TaskHelper.EmptyTask get-only property and we can make SystemClock an internal class if they weren't in the shipped api.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.38%. Comparing base (9f004a2) to head (7a1a2b8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2316   +/-   ##
=======================================
  Coverage   85.38%   85.38%           
=======================================
  Files         313      313           
  Lines        7457     7457           
  Branches     1126     1126           
=======================================
  Hits         6367     6367           
  Misses        745      745           
  Partials      345      345           

Flag	Coverage Δ
linux	85.38% <ø> (+0.02%)	⬆️
macos	85.38% <ø> (ø)
windows	85.38% <ø> (+0.02%)	⬆️

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