Bump dotnet

This commit fixes the CI error on `main` branch where
amqp_system_SUITE failed with the following error:
```
Process terminated. Couldn't find a valid ICU package installed on the system. Set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support.
   at System.Environment.FailFast(System.String)
   at System.Globalization.GlobalizationMode.GetGlobalizationInvariantMode()
   at System.Globalization.GlobalizationMode..cctor()
   at System.Globalization.CultureData.CreateCultureWithInvariantData()
   at System.Globalization.CultureData.get_Invariant()
   at System.Globalization.CultureInfo..cctor()
   at System.String.ToLowerInvariant()
   at Microsoft.DotNet.PlatformAbstractions.RuntimeEnvironment.GetArch()
   at Microsoft.DotNet.PlatformAbstractions.RuntimeEnvironment..cctor()
   at Microsoft.DotNet.PlatformAbstractions.RuntimeEnvironment.GetRuntimeIdentifier()
   at Microsoft.DotNet.Cli.MulticoreJitProfilePathCalculator.CalculateProfileRootPath()
   at Microsoft.DotNet.Cli.MulticoreJitActivator.StartCliProfileOptimization()
   at Microsoft.DotNet.Cli.MulticoreJitActivator.TryActivateMulticoreJit()
   at Microsoft.DotNet.Cli.Program.Main(System.String[])

Exit code: 134 (pid <0.1533.0>)
```

Author: ansd

.github/workflows/test-make-target.yaml

@@ -45,7 +45,7 @@ jobs:
       uses: actions/setup-dotnet@v4
       if: inputs.plugin == 'rabbit'
       with:
-        dotnet-version: '3.1.x'
+        dotnet-version: '8.0'
 
     - name: SETUP SLAPD (rabbitmq_auth_backend_ldap)
       if: inputs.plugin == 'rabbitmq_auth_backend_ldap'

.github/workflows/test-plugin.yaml

@@ -75,7 +75,7 @@ jobs:
     - uses: actions/setup-dotnet@v4
       if: inputs.plugin == 'rabbit'
       with:
-        dotnet-version: '3.1.x'
+        dotnet-version: '8.0'
     - name: deps/amqp10_client SETUP
       if: inputs.plugin == 'amqp10_client'
       run: |

deps/rabbit/test/amqp_system_SUITE_data/fsharp-tests/global.json

@@ -1,5 +1,5 @@
 {
   "sdk": {
-    "version": "3.1"
+    "version": "8.0"
   }
 }