replace toLocalString with toISOSTring

[lassopicasso]

Description

Make sure consistent time format. The test was failing locally on my computer

Related Issue(s)

• itself

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

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

[mlqn]

💯

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.18%. Comparing base (eaca3da) to head (0933283).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12700   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files        1299     1299           
  Lines       18573    18573           
  Branches     2261     2261           
=======================================
  Hits        16750    16750           
  Misses       1564     1564           
  Partials      259      259           

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