chore(testing): NuGet updates, dead code removal

[tippmar-nr]

• Updates Serilog to v3.0.1 (from 2.12.2) which should bring some performance improvements and general goodness. See Serilog v3.0.0 for details.
• Updates Core.csproj ILRepack configuration to move packages used in both net462 and netstandard2.0 to the "common" configuration rather than repeating them in the tfm-specific sections.
• Updates several NuGet packages (nUnit, xUnit, coverlet.collector, etc.) used in our unit, integration and unbounded test projects.
• Fixes unit tests in NewRelic.Testing.Assertions.UnitTests that were broken with the update to the latest nUnit version.
• Removes NoOpAttributeValueCollection class as it is unused anywhere.

[codecov-commenter]

Codecov Report

Merging #2026 (1c0e7bf) into main (14c6bb1) will increase coverage by 0.07%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2026      +/-   ##
==========================================
+ Coverage   80.22%   80.30%   +0.07%     
==========================================
  Files         404      404              
  Lines       24927    24903      -24     
  Branches     3003     3003              
==========================================
  Hits        19998    19998              
+ Misses       4143     4119      -24     
  Partials      786      786              

Files	Coverage Δ
.../Agent/Core/Attributes/AttributeValueCollection.cs	82.59% <ø> (+8.01%)	⬆️

... and 2 files with indirect coverage changes