Upgrade code coverage job to net 8.

[codecov[bot]]

Codecov Report

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

Please upload report for BASE (main@fc0acc0). Learn more about missing BASE report. Report is 13 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2945 +/- ## ======================================= Coverage ? 72.23% ======================================= Files ? 291 Lines ? 59709 Branches ? 0 ======================================= Hits ? 43133 Misses ? 16576 Partials ? 0 ``` | [Flag](https://app.codecov.io/gh/dotnet/SqlClient/pull/2945/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/2945/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/2945/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.37% <ø> (?)` | | | [netfx](https://app.codecov.io/gh/dotnet/SqlClient/pull/2945/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `70.60% <ø> (?)` | | 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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[benrr101]