Change version of .Net in dev container to 8 so it can run Polyglot n… (

microsoft#4201)

…otebooks

### Motivation and Context
It using a dev container, we currently can't run Polyglot notebooks
since they required .Net 8

### Description
Update .Net version in dev container from 7 to 8

### Contribution Checklist
- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

.devcontainer/devcontainer.json

@@ -3,7 +3,7 @@
 	"features": {
 		"ghcr.io/devcontainers/features/node:1": {},
 		"ghcr.io/devcontainers/features/dotnet:1": {
-			"version": "7"
+			"version": "8"
 		},
 		"ghcr.io/jlaundry/devcontainer-features/azure-functions-core-tools:1": {}
 	},