Remove dependency on EF Core "Issue26779" AppContext switch

[bkoelman]

EF Core v6.0.2 introduced a breaking change regarding change tracking of many-to-many relationships, which broke removing from many-to-many relationships in JsonApiDotNetCore in some cases. Fortunately, an AppContext switch to revert to the old behavior was added, which we've used since JsonApiDotNetCore v5.0.0.

The switch has been removed in EF (Core) 7. This PR fixes our code to no longer require the "Microsoft.EntityFrameworkCore.Issue26779" backward-compatibility switch. This unblocks us from upgrading to the latest preview version.

QUALITY CHECKLIST

• [x] Changes implemented in code
• [x] Complies with our contributing guidelines
• [x] Adapted tests
• [ ] N/A: Documentation updated

[codecov[bot]]

Codecov Report

Merging #1176 (6825c83) into master (0185311) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1176   +/-   ##
=======================================
  Coverage   92.58%   92.59%           
=======================================
  Files         241      241           
  Lines        7700     7709    +9     
=======================================
+ Hits         7129     7138    +9     
  Misses        571      571           

Impacted Files	Coverage Δ
.../JsonApiDotNetCore/Configuration/JsonApiOptions.cs	100.00% <ø> (ø)
...Core/Repositories/EntityFrameworkCoreRepository.cs	99.31% <100.00%> (+0.02%)	:arrow_up:

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