🌐 i18n(es): Added plural forms and gift duration labels (#28054)

Refs: #23361

### What

- Translate plural forms `{count} month_one/other` and `{count}
year_one/other` (used in the gift subscription Open Graph preview).
- Translate `one-month` and `one-year` duration labels (used in the gift
purchase confirmation email, as adjectives in "a one-month / one-year
membership").
- Remove obsolete `{count} month_many` and `{count} year_many` keys:
Spanish CLDR plural rules only use `_one` and `_other`, so the `_many`
forms were never reachable and they no longer exist in `en.json`.

### Translations

| Key | Value |
|---|---|
| `{count} month_one` | `{count} mes` |
| `{count} month_other` | `{count} meses` |
| `{count} year_one` | `{count} año` |
| `{count} year_other` | `{count} años` |
| `one-month` | `un mes` |
| `one-year` | `un año` |