TargetFrameworks and .csproj file optimizations

[Seramis]

Description

• netstandard2.0 uses Microsoft.Bcl.Memory now - was able to remove the compatibility files for Span index and range functionality.
• Moved the common .csproj config of test projects into Directory.Build.props file.
• Adjusted LinqToDB TFM list - now supports netstandard2.0+ and aligns better with official linq2db nuget.
• Fixed small warning in some tests.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Other (please specify): Optimizations

Checklist

• The PR is submitted to the correct branch (main).
• My code follows the project's coding style. (.editorconfig)
• I have commented my code, particularly in hard-to-understand areas and public interfaces.
• I have added or updated tests for the changes I made.
• All new and existing tests passed.
• I have updated the documentation where applicable.