minor symfony#53197 [Validator] Add missing Serbian translation (marac19901990)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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[Validator] Add missing Serbian translation

[…ions](symfony#53043)

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | Fix symfony#53043
| License       | MIT

Added one new missing translation, and reformulated the wording on old ones. This is my native language so i wanted to make it as clear as possible.

Commits
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bdee377 [Validator] Add missing Serbian translation

Author: nicolas-grekas