Changes - Updated DetermineExpiryDate function signature in...

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Changes

- Updated DetermineExpiryDate function signature in utils/expiry_date.go

- Updated all 5 token resources to handle the new return type:

• resource_gitlab_project_access_token.go (3 locations)

- resource_gitlab_group_access_token.go (3 locations)

• resource_gitlab_personal_access_token.go (3 locations)

- resource_gitlab_user_impersonation_token.go (1 location)

- resource_gitlab_group_service_account_access_token.go (3 locations)

- Returned both gitlab.ISOTime and string.type for validation.

- Updated unit tests to work with new return type

- Added test case for unknown value scenario closes #6617 (closed)