[1.x] Allow guzzlehttp/promises ^3.0 (#536)

Guzzle 8 requires guzzlehttp/promises ^3.0.1, but Pulse currently caps it
at ^1.0|^2.0. Since laravel/framework already permits guzzle ^8.0, that cap
is the remaining blocker preventing any Laravel 13.26+ application that uses
Pulse from upgrading to Guzzle 8.

Pulse's only promises touchpoint is `new RejectedPromise($exception)` in
src/Recorders/SlowOutgoingRequests.php, and that constructor is unchanged
between promises 2.5.2 and 3.0.1.

The require-dev bump to guzzle ^7.7|^8.0 is what allows CI to actually
resolve promises 3.x — without it guzzle ^7.7 pins promises back to 2.x and
the new constraint would never be exercised.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>