PO Gettext extract does not generate correct plural placeholders

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### Description

When running `lingui extract` with locales that have different plural rules, we would expect the PO to have the correct number of `msgstr` for the number of plurals, but instead it just uses two (source locale, not even source code. Even if your source code has `zero, one, many` it will only put `msgstr[0], msgstr[1]`

### Verifications

- [x] I've checked [the docs](https://lingui.dev) and this isn't covered there.
- [x] I've searched existing issues on [GitHub](https://github.com/lingui/js-lingui/issues).

### Reproduction Steps

Add any plural

```tsx

And use `ru` in locales - good example, as it has 4 plural forms, and rule is:
```
"Plural-Forms: nplurals=4; plural=(n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<12 || n%100>14) ? 1 : n%10==0 || (n%10>=5 && n%10<=9) || (n%100>=11 && n%100<=14)? 2 : 3);\n"
```

### Expected Behavior

Expect to see:

```po
#. js-lingui:icu=%7Bcount%2C+plural%2C+one+%7BThere%27s+one+book%7D+other+%7BThere+are+%23+books%7D%7D&pluralize_on=count
#: src/App.tsx:52
msgid "There's one book"
msgid_plural "There are # books"
msgstr[0] ""
msgstr[1] ""
msgstr[2] ""
msgstr[3] ""
```

Instead only get

```po
#. js-lingui:icu=%7Bcount%2C+plural%2C+one+%7BThere%27s+one+book%7D+other+%7BThere+are+%23+books%7D%7D&pluralize_on=count
#: src/App.tsx:52
msgid "There's one book"
msgid_plural "There are # books"
msgstr[0] ""
msgstr[1] ""
```

### Macro Support

Babel with babel-macro-plugin

### Lingui Version

5.5.1

### Babel Version

_No response_

### Framework

_No response_

Start by reproducing the issue with `lingui extract`, the provided plural examples, and the `ru` locale. Trace how plural metadata becomes PO `msgstr` entries; done means the generated PO contains four empty plural translations matching the locale's plural rules instead of two.

Written by the indexing model from the issue text.