save old strings as old and not as current

[timse201]

trac ticket: 6320

If we sync strings from dev to stable, the overwritten string in stable will fire gp_translation_prepare_for_save again (if I'm right). Because there is no check for old strings, it will become current again, which leads to 2 new current strings in stable.
This PR adds a check for status old.

[github-actions]

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Props timse201.

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