test: .NET 8 container test now tests a 2.x.x version of EnyimMemcachedCore

[tippmar-nr]

Dotty was never configured to ignore the .NET 8 TFM for EnyimMemcachedCore, so we've been testing only the 3.x version since late 2024 when Dotty first ran the automated update PR.

This PR restores testing of a 2.x version on the .NET 8 TFM and updates Dotty to ignore the .NET TFM for that package when making updates.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.77%. Comparing base (e2233e6) to head (c34c425).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3513   +/-   ##
=======================================
  Coverage   81.77%   81.77%           
=======================================
  Files         508      508           
  Lines       34220    34220           
  Branches     4040     4040           
=======================================
  Hits        27984    27984           
  Misses       5269     5269           
  Partials      967      967           

Flag	Coverage Δ
Agent	82.76% <ø> (ø)
Profiler	71.75% <ø> (ø)

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