[13.x] Fix incorrect @return types in Number::spell(), ordinal(), and spellOrdinal()

[AmdadulShakib]

Changes

Fix incorrect @return type annotations on three methods in the Number class.

According to the PHP documentation,
NumberFormatter::format() returns string|false — it returns false on failure.

The following methods call NumberFormatter::format() internally but their
docblocks incorrectly promised only string:

• Number::spell() — when $number <= $after or $number >= $until, it falls
  through to static::format() which can return false
• Number::ordinal() — calls $formatter->format($number) directly
• Number::spellOrdinal() — calls $formatter->format($number) directly

This causes incorrect static analysis results (PHPStan/Psalm) when these return
values are used as strings without null/false checks.

[shaedrich]

I wonder whether we want to return false or actually do something else when the underlying NumberFormatter returns false