chore: Require Guzzle 7.15.2 or higher

[stephangroen]

Summary

• Require guzzlehttp/guzzle: ^7.15.2 instead of ~6.0|~7.0.
  • Guzzle 6 and Guzzle 7 below 7.15.2 have known security advisories. These are the 9 open Dependabot alerts: 1 high, 8 medium.
  • Modern Composer already refuses to install any Guzzle 6 version, because of the security advisories. Guzzle 6 support was untested and effectively unusable.
  • Guzzle 7.15.2 still supports PHP 7.4, so the supported PHP versions don't change.
• CI now also runs the tests with --prefer-lowest on PHP 7.4 and 8.5, so the minimum versions are actually tested. The existing job names (PHP 7.4 … PHP 8.5) stay the same, so the required status checks keep working.
• The README's "Guzzle versions" section points Guzzle 6 users to v4.7.

Intended for v4.8.0. Users still on Guzzle 6 or an older Guzzle 7 can keep using v4.7.

Test plan

• Lowest dependencies (Guzzle 7.15.2, psr7 2.13.0): 592 tests pass, composer audit finds no advisories
• Highest dependencies: 592 tests pass
• CI passes, including the new "lowest dependencies" jobs

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