Fix #31111: missing custom metadata after upgrade.

[thibsy]

Hello @smeyer-ilias, as described in https://mantis.ilias.de/view.php?id=31111 an update step is missing that migrates data from the database-table adv_md_values_text to adv_md_values_ltext. This PR introduces that missing step.

I made an assumption though, because the new table adv_md_values_ltext contains another column called value_index, where I just adpoted the default lang-code from adv_md_field_int because that information previously didn't exist.

Is it also necessary to create an entry with and without value_index? because that is the default behaviour when adding custom-metadata values per GUI in ILIAS 7 and I didn't know if that's intended.