chore: fix security vulnerability for slacknotifier (#815)

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## Description

<!--- Describe your changes in detail -->

Got notified about the issue in GH security scanning:
https://github.com/digdir/dialogporten/security/code-scanning/5

## Related Issue(s)

- #{issue number}

## Verification

- [ ] **Your** code builds clean without any errors or warnings
- [ ] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)

.azure/modules/functionApp/slackNotifier.bicep

@@ -25,6 +25,7 @@ resource storageAccount 'Microsoft.Storage/storageAccounts@2023-04-01' = {
     properties: {
         supportsHttpsTrafficOnly: true
         defaultToOAuthAuthentication: true
+        minimumTlsVersion: 'TLS1_2'
     }
 }