Multi-target against .NET 6 and .NET 8

[bkoelman]

This PR targets the RTM versions of .NET / EF Core v8.

It currently references nightly builds of Npgsql.EntityFrameworkCore.PostgreSQL and Pomelo.EntityFrameworkCore.MySql because there are no stable releases yet that are compatible with EF Core 8.

Blocked by https://youtrack.jetbrains.com/issue/RSRP-494775/InspectCode-CleanupCode-hangs-on-.NET-8-RTM.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (c1b911f) 90.79% compared to head (9546bcb) 90.81%.

❗ Current head 9546bcb differs from pull request most recent head dea63fd. Consider uploading reports for the commit dea63fd to get more accurate results

Files	Patch %	Lines
...onApiDotNetCore.Annotations/CollectionConverter.cs	62.50%	2 Missing and 1 partial ⚠️
...re/Queries/QueryableBuilding/WhereClauseBuilder.cs	80.00%	1 Missing and 1 partial ⚠️
src/Examples/DapperExample/SystemClock.cs	0.00%	1 Missing ⚠️
...sonApiDotNetCore/Diagnostics/CascadingCodeTimer.cs	0.00%	1 Missing ⚠️
...etCore/QueryStrings/FieldChains/MatchTraceScope.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1349      +/-   ##
==========================================
+ Coverage   90.79%   90.81%   +0.01%     
==========================================
  Files         342      342              
  Lines       11069    11058      -11     
  Branches     1811     1812       +1     
==========================================
- Hits        10050    10042       -8     
- Misses        667      668       +1     
+ Partials      352      348       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.