test: Remove net481 build targets, [NetFrameworkTest] / [NetCoreTest] attributes from integration test projects

[tippmar-nr]

• Removes the unnecessary net481 build target from Integration and Unbounded test projects and related helper projects (where possible - some projects are shared with unit tests and still need to multi-target).
• Removes [NetFrameworkTest] and [NetCoreTest] attributes that aren't used for anything. This change comprises the vast majority of file updates in this PR.
• Updates all_solutions workflow to reference the net9.0 build folder instead of net481 for integration test build path

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.30%. Comparing base (12c8288) to head (2636191).
⚠️ Report is 291 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3093   +/-   ##
=======================================
  Coverage   82.30%   82.30%           
=======================================
  Files         477      477           
  Lines       30636    30636           
  Branches     3424     3424           
=======================================
+ Hits        25214    25216    +2     
+ Misses       4622     4621    -1     
+ Partials      800      799    -1     

Flag	Coverage Δ
Agent	83.37% <ø> (+<0.01%)	⬆️
Profiler	72.51% <ø> (ø)

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see 1 file with indirect coverage changes

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