Cleanup the `Polly` and `Polly.Specs` codebase

[martintmk]

In the #1287 we enabled a full set of analyzers for these projects. Many of the rules are currently suppressed in each respective csproj file with the <NoWarn> element.

We can enable these rules one-be-one and cleanup the codebase.

[davidCett]

Hi👋I would like to help with this issue

[martincostello]

@davidCett Thanks for volunteering! Feel free to open a pull request to tackle one of the disabled warnings. I suggest you tackle one or two per pull request to prevent the diffs getting too big to easily review.