renamed validators.ua.xlf to validators.uk.xlf

ISO 639-1 code for Ukrainian is "uk"
bug #9674 [2.2][Form] rename validators.ua.xlf to validators.uk.xlf (craue)

This PR was submitted for the 2.2 branch but it was merged into the 2.3 branch instead (closes #9674).

Discussion
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[2.2][Form] rename validators.ua.xlf to validators.uk.xlf

| Q             | A
| ------------- | ---
| Fixed tickets | --
| License       | MIT

The ISO 639-1 code for Ukrainian is "uk".

Shall I open separate PRs for newer branches as well, or can this be merged into them?

Commits
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fe6995d renamed validators.ua.xlf to validators.uk.xlf