Don't keep file streams open

[ivarne]

There have been reported issues with the new PATCH endpoint in localtest with error messages like

fail: Altinn.App.Api.Controllers.DataController[0]
      Unable to update data element 57abc10b-d54d-4fd8-bcee-79876d3feaee for instance 500000/8125c8ac-7624-44f3-8429-37ab780aa86c
      Altinn.App.Core.Helpers.PlatformHttpException: 500 - Internal Server Error - System.IO.IOException: The process cannot access the file '/AltinnPlatformLocal/blobs/krt/krt-1185a-1/8125c8ac-7624-44f3-8429-37ab780aa86c/data/57abc10b-d54d-4fd8-bcee-79876d3feaee' because it is being used by another process.

This is likely caused by not closing streams.

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)
• All tests run green

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)