Add support for Guzzle 8

[GrahamCampbell]

This adds Guzzle 8 support by widening the guzzlehttp/guzzle, guzzlehttp/psr7 and guzzlehttp/promises constraints to allow their new majors alongside the current ones. Guzzle 8 removed getResponse() from the base RequestException, where it now lives only on the ResponseException subclasses, and reclassified connection failures under Psr\Http\Client\NetworkExceptionInterface. The five API exception converters therefore guard the response lookup so it resolves on both majors, and the Remote Config converter catches the PSR-18 network interface for connection errors. The behaviour is unchanged on Guzzle 7.

Composer can only resolve Guzzle 8 here once this project's own dependencies allow it, so this PR depends on releases from beste/firebase-tokens-php#73 (kreait/firebase-tokens), googleapis/google-auth-library-php#677 (google/auth) and googleapis/google-cloud-php#9377 (google/cloud-storage). Until those ship, Composer keeps resolving Guzzle 7 and this branch stays fully compatible with it.

[jeromegamez]

PR 1111, nice! Thank you for your work - I'm keeping track of the PRs in the other repos and will enable the workflows once they're all ready.

I believe https://github.com/googleapis/gax-php needs the same treatment, too. My mistake, it's been incorporated into the cloud core project since I last looked 😅

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (d8268a3) and HEAD (d7dc96c). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (d8268a3)	HEAD (d7dc96c)
grpc	3	0
emulator	3	0
integration	3	0

@@              Coverage Diff              @@
##                8.x    #1111       +/-   ##
=============================================
- Coverage     87.80%   40.10%   -47.70%     
- Complexity     1486     1491        +5     
=============================================
  Files           146      146               
  Lines          4189     4174       -15     
=============================================
- Hits           3678     1674     -2004     
- Misses          511     2500     +1989     

[jeromegamez]

Thanks again, much appreciated!