#5955 Use .toISOString() in compatibility.ts

[martgil]

This PR updates the compatibility.ts output of the key's expiration field to be formatted in ISO Format rather than in Epoch time for quick recognition of the expiry date.

Closes #5955

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Tests (delete all except exactly one):

• Tests added or updated

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[martgil]

Hi @sosnovsky - This one is ready for a review as well. Thanks!

[sosnovsky]

looks good 👍