Remove Moq

[martincostello]

• Remove Moq and replace with NSubstitute.
• Fix a bug masked by Moq where we were sending empty buckets.
• Use C#10, rather than 11, to resolve warnings I don't know how to fix related to the new scoped keyword (see Update to .NET 7 #374 (comment)).

[codecov]

Codecov Report

Merging #531 (9a02163) into main (b5a1302) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #531      +/-   ##
==========================================
- Coverage   95.04%   94.88%   -0.17%     
==========================================
  Files          27       27              
  Lines         606      606              
==========================================
- Hits          576      575       -1     
- Misses         30       31       +1     

Flag	Coverage Δ
linux	94.55% <100.00%> (-0.50%)	⬇️
macos	94.55% <100.00%> (-0.34%)	⬇️
windows	94.05% <100.00%> (ø)

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Files Changed	Coverage Δ
...tEat.StatsD/Buffered/BufferBasedStatsDPublisher.cs	93.93% <100.00%> (ø)

... and 1 file with indirect coverage changes

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