fix: updates devcontainer to dotnet 8 (Azure#1136)

## Description

- Bicep is not compatible anymore with dotnet 7. Updating the
devcontainer to dotnet 8.

    ```bash
error NU1202: Package Azure.Bicep.RegistryModuleTool 0.25.53 is not
compatible with net7.0 (.NETCoreApp,Version=v7.0) / any. Package
Azure.Bicep.RegistryModuleTool 0.25.53 supports: net8.0
(.NETCoreApp,Version=v8.0)
    ```

- adding Az.Accounts, Az.Resources and Az.KeyVault for module testing

Fixes Azure#1135 

## Pipeline Reference

## Type of Change

- [ ] Update to CI Environment or utlities (Non-module effecting
changes)
- [ ] Azure Verified Module updates:
- [ ] Bugfix containing backwards compatible bug fixes, and I have NOT
bumped the MAJOR or MINOR version in `version.json`:
- [ ] Someone has opened a bug report issue, and I have included "Closes
#{bug_report_issue_number}" in the PR description.
- [x] The bug was found by the module author, and no one has opened an
issue to report it yet.
- [ ] Feature update backwards compatible feature updates, and I have
bumped the MINOR version in `version.json`.
- [ ] Breaking changes and I have bumped the MAJOR version in
`version.json`.
  - [ ] Update to documentation

## Checklist

- [x] I'm sure there are no other open Pull Requests for the same
update/change
- [ ] I have run `Set-AVMModule` locally to generate the supporting
module files.
- [ ] My corresponding pipelines / checks run clean and green without
any errors or warnings

---------

Co-authored-by: Sebastian Gräf <sebastian@graef.io>
Co-authored-by: ChrisSidebotham-MSFT <48600046+ChrisSidebotham@users.noreply.github.com>

.devcontainer/devcontainer.json

@@ -17,14 +17,14 @@
       ]
     }
   },
-  "postCreateCommand": "npm install -g prettier && dotnet tool install -g Azure.Bicep.RegistryModuleTool",
+  "postCreateCommand": "npm install -g prettier && dotnet tool install -g Azure.Bicep.RegistryModuleTool && pwsh -C Install-Module Az.Accounts -Repository PSGallery -Force && pwsh -C Install-Module Az.Resources -Repository PSGallery -Force && pwsh -C Install-Module Az.KeyVault -Repository PSGallery -Force",
   "postStartCommand": "az bicep install",
   "remoteEnv": {
     "PATH": "${containerEnv:PATH}:/home/node/.dotnet/tools:/home/node/.azure/bin"
   },
   "features": {
     "azure-cli": "latest",
-    "dotnet": "7.0",
+    "dotnet": "8.0",
     "ghcr.io/devcontainers/features/node:1": {},
     "ghcr.io/devcontainers/features/powershell:1": {}
   }