Drop Carbon dependency, replace with DateTimeImmutable

[vicdelfant]

With Carbon's long-awaited 3.0 release being out the door 🎉, I discovered that the Oh Dear SDK was blocking the upgrade of one of our internal libraries because of its dependency on nesbot/carbon:^2.65.0.

As it turns out, however, Carbon is only being used by one method:

ohdear-php-sdk/src/OhDear.php

Lines 81 to 84 in d306069

	public function convertDateFormat(string $date, $format = 'YmdHis'): string
	{
	return Carbon::parse($date)->format($format);
	}

With this in mind, it seemed more efficient to just use the native DateTimeImmutable instead of depending on an external library, especially with a new major release of it being out in the wild now.

This PR drops the dependency on Carbon and replaces it with a simple call to DateTimeImmutable::format(), while keeping green checks in the tests.

[freekmurze]

Perfect, thank you!