Simplify pre-processor conditions

[martincostello]

Use #if NET rather than including the version.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.78%. Comparing base (627b436) to head (f2dec50).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #683 +/- ## ======================================= Coverage 97.78% 97.78% ======================================= Files 15 15 Lines 271 271 Branches 37 37 ======================================= Hits 265 265 Misses 3 3 Partials 3 3 ``` | [Flag](https://app.codecov.io/gh/martincostello/xunit-logging/pull/683/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Costello) | Coverage Δ | | |---|---|---| | [linux](https://app.codecov.io/gh/martincostello/xunit-logging/pull/683/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Costello) | `97.04% <ø> (ø)` | | | [macos](https://app.codecov.io/gh/martincostello/xunit-logging/pull/683/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Costello) | `97.78% <ø> (?)` | | | [windows](https://app.codecov.io/gh/martincostello/xunit-logging/pull/683/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Costello) | `?` | | 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=Martin+Costello#carryforward-flags-in-the-pull-request-comment) to find out more.

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