[12.x] Allow validator message placeholers to be capitalized

[florianraith]

Extend placeholder capitalization beyond :attribute to other validator placeholders (e.g., :other in the same rule). This makes behavior consistent across placeholders and improves localization for languages such as German, where field names are typically capitalized.

// Example
'same' => 'The :Attribute field must match :Other.',

// Before: "The Password field must match :Other."
// After:  "The Password field must match Password confirmation."

This PR addresses the inconsistency where :Attribute was supported, but other placeholders (like :Other) were not.

The implementation is inspired by FormatMessages::replaceAttributePlaceholder() which implements the feature for :attribute

[shaedrich]

Would it make sense to move that logic to its own method and then reuse it?

private function replaceKeepCase($message, $placeholder, $parameter)
{
	$fn = [ Str::lower(...), Str::upper(...), Str::ucfirst(...) ];
	$cases = array_map(fn (callable $fn) => ':' . $fn($placeholder), $fn);
	$replacements = array_map(fn (callable $fn) => $fn($parameter), $fn);
	return str_replace($cases, $replacements, $message);
}

protected function replaceAcceptedIf($message, $attribute, $rule, $parameters)
{
    $parameters[1] = $this->getDisplayableValue($parameters[0], Arr::get($this->data, $parameters[0]));

    $parameters[0] = $this->getDisplayableAttribute($parameters[0]);

    return $this->replaceKeepCase(
		$this->replaceKeepCase($message, 'other', $parameters[0]),
		'value',
		$parameters[1],
	);
}