Add support for Guzzle 8

[HecFranco]

Summary

• Allow guzzlehttp/guzzle ^8.0 alongside ^7.0 so consumers can upgrade their HTTP client stack without a fork or constraint conflict.
• No runtime API changes; this is a Composer constraint-only update.

Context

Guzzle 8 is available and some projects (including ours) are moving to it. The package currently requires ^7.0 only, which blocks that upgrade path even though the library usage of Guzzle remains compatible.

Changes

• composer.json: guzzlehttp/guzzle → ^7.0 || ^8.0

Test plan

• Install with Guzzle 7 (composer update guzzlehttp/guzzle:^7.0) and run the test suite
• Install with Guzzle 8 (composer update guzzlehttp/guzzle:^8.0) and run the test suite
• Smoke a simple GoogleTranslate::trans(...) / instance translate() call under both major versions

[HecFranco]

Hi maintainers 👋

First of all, thank you for maintaining this package — it has been very useful for us.

We’ve opened this PR to add Guzzle 8 support by relaxing the Composer constraint to ^7.0 || ^8.0. There are no code/API changes; it should remain fully compatible with Guzzle 7 while unblocking projects that have already moved (or are moving) to Guzzle 8.

Would you mind reviewing and approving this MR when you have a moment? Happy to adjust anything if needed.

Thanks again for your work on google-translate-php!

[Stichoza]

Thanks for your PR! 🙌🏻