Use random shared

[leotsarev]

Advantages:

• Do not allocate Random & GlobalRandom instances on .NET 6
• Common Random implementation in Random() and other places

[codecov]

Codecov Report

Merging #907 (0359123) into master (4e09ca4) will increase coverage by 0.00%.
The diff coverage is 71.42%.

@@           Coverage Diff           @@
##           master     #907   +/-   ##
=======================================
  Coverage   92.39%   92.39%           
=======================================
  Files         111      112    +1     
  Lines        3443     3444    +1     
  Branches     1021     1021           
=======================================
+ Hits         3181     3182    +1     
  Misses        200      200           
  Partials       62       62           

Impacted Files	Coverage Δ
MoreLinq/Random.cs	91.89% <ø> (+6.47%)	⬆️
MoreLinq/GlobalRandom.cs	66.66% <66.66%> (ø)
MoreLinq/RandomSubset.cs	100.00% <100.00%> (ø)
MoreLinq/Shuffle.cs	100.00% <100.00%> (ø)

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[atifaziz]

Thanks for this!

I've made some minor changes:

• Reduced the scope of conditional parts within GlobalRandom.
• Fixed the doc comment (anchor, grammar and formatting).
• Conditionally imported System.Threading on .NET 6+.
• Added the note of using Random.Shared on .NET 6+ to the various Random* methods.
• Undid the move of GlobalRandom into a separate file since it's an unrelated change that can go into a follow-up PR (if absolutely necessary) and at which point the type could be un-nested too.

The net effect is a smaller diff.