Clean up Preprocessor Directives After NET6 Removal

[mdaigle]

Building on #2927. That PR should merge first.

This PR includes a few different types of preprocesser directive cleanups:

• in netcore, removing #if NET6_0_OR_GREATER and #if NET8_0_OR_GREATER
  • these always evaluated to true
• in netcore, removing #if NETFRAMEWORK
  • these always evaluated to false
• in netfx, removing #if NETX_0_OR_GREATER
  • these always evaluated to false
• in common and test, replacing #if NETX_0_OR_GREATER with #if NET
  • for readability, these didn't actually care what version of .NET was being used.

[ErikEJ]

Looks like this includes your previous PR?

[mdaigle]

Looks like this includes your previous PR?

@ErikEJ yes, this is building on the previous PR. I'll move it out of draft when the previous PR merges.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@fc0acc0). Learn more about missing BASE report. Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/Microsoft/Data/SqlClient/Server/SqlNormalizer.cs	0.00%	1 Missing :warning:
....SqlClient/src/Microsoft/Data/SqlClient/SqlUtil.cs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2938 +/- ## ======================================= Coverage ? 72.21% ======================================= Files ? 291 Lines ? 59709 Branches ? 0 ======================================= Hits ? 43120 Misses ? 16589 Partials ? 0 ``` | [Flag](https://app.codecov.io/gh/dotnet/SqlClient/pull/2938/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | Coverage Δ | | |---|---|---| | [addons](https://app.codecov.io/gh/dotnet/SqlClient/pull/2938/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `92.58% <ø> (?)` | | | [netcore](https://app.codecov.io/gh/dotnet/SqlClient/pull/2938/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.34% <85.71%> (?)` | | | [netfx](https://app.codecov.io/gh/dotnet/SqlClient/pull/2938/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `70.60% <85.71%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ErikEJ]

LGTM