Use dotnet 8.x in pre-commits to fix flaky failures (#6083)

The 8.x release should contain dotnet/runtime#90342 which fixes dotnet/runtime#80619.

I hope this will fix flaky failures like https://github.com/Unity-Technologies/ml-agents/actions/runs/8268945605/job/22623023348 of the form:
```
dotnet-format............................................................Failed
- hook id: dotnet-format
- exit code: 1

System.IO.IOException: The system cannot open the device or file specified. : 'NuGet-Migrations'
   at System.Threading.Mutex.CreateMutexCore(Boolean initiallyOwned, String name, Boolean& createdNew)
   at System.Threading.Mutex..ctor(Boolean initiallyOwned, String name)
   at NuGet.Common.Migrations.MigrationRunner.Run(String migrationsDirectory)
   at Microsoft.DotNet.Configurer.DotnetFirstTimeUseConfigurer.Configure()
   at Microsoft.DotNet.Cli.Program.ConfigureDotNetForFirstTimeUse(IFirstTimeUseNoticeSentinel firstTimeUseNoticeSentinel, IAspNetCertificateSentinel aspNetCertificateSentinel, IFileSentinel toolPathSentinel, Boolean isDotnetBeingInvokedFromNativeInstaller, DotnetFirstRunConfiguration dotnetFirstRunConfiguration, IEnvironmentProvider environmentProvider, Dictionary`2 performanceMeasurements)
   at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, TimeSpan startupTime, ITelemetry telemetryClient)
   at Microsoft.DotNet.Cli.Program.Main(String[] args)
```

.github/workflows/pre-commit.yml

@@ -25,7 +25,7 @@ jobs:
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
     - uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '7.0.100'
+        dotnet-version: '8.0.202'
     - name: Install manual dependencies
       run: |
         python -m pip install pre-commit