Only publish coverage against one target framework

[nulltoken]

Pull request description

Coverage reports seem... a bit unstable (see #114 (comment) for instance).

Currently the tests are run for both net8.0 and net9.0. As such two different coverage reports are submitted to coveralls.
Maybe this dual reporting is what generates the "instability".

This proposed change runs the tests once again, for the sole purpose of coverage analysis, but by only targeting net8.0.

PR meta checklist

• Pull request is targeted at main branch for code
• Pull request is linked to all related issues, if any.

Code PR specific checklist

• My code follows the code style of this project and AspNetCore coding guidelines.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have updated the appropriate sub section in the CHANGELOG.md.
• I have added, updated or removed tests to according to my changes.
  • All tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 11627901069

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.5%) to 84.104%

---

Files with Coverage Reduction	New Missed Lines	%
src/NCronJob/Scheduler/ObservablePriorityQueue.cs	1	91.38%
src/NCronJob/Scheduler/JobProcessor.cs	3	73.08%

Totals
Change from base Build 11627027565:	-0.5%
Covered Lines:	1025
Relevant Lines:	1170

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💛 - Coveralls

[nulltoken]

This seems to clear up some fog and (hopefully) should provide a less fluctuating baseline.

Before:

With this change:

[linkdotnet]

Interesting. Until now we don't have any. net9.0 exclusive stuff (besides System.Threading.Lock). So let's roll with it