Require patched Guzzle and PHPUnit versions, and prepare 3.0.1

[MudassarTariq]

What

This raises the minimum versions of two dependencies so installs can no longer pick releases with known security vulnerabilities. It also prepares release 3.0.1.

Package	Constraint
guzzlehttp/guzzle	^7.8 → ^7.15.2
phpunit/phpunit (dev)	^10.5 → ^10.5.62

• No code changes. The release commit bumps Version::VERSION (used in the User-Agent) to 3.0.1 and adds a CHANGELOG entry.
• php ^8.1 is unchanged. Guzzle 7.15.2 supports PHP 7.2.5+ and 8.x, and PHPUnit 10.5.62 supports PHP 8.1+.

Security

• Guzzle: every release below 7.15.2 has at least one of 9 advisories: CVE-2026-55568, CVE-2026-55767, CVE-2026-59883, CVE-2026-67339, CVE-2026-67353, CVE-2026-67354, CVE-2026-67355, CVE-2026-69245 and CVE-2026-69246 (one high: noncanonical host can bypass host-based checks). 7.8.0, the old minimum, has all 9.
• PHPUnit: releases below 10.5.62 on the 10.x line have CVE-2026-24765.
• Current Composer (2.9+) already refuses versions with known advisories, but older Composer versions and dependency scanners go by the declared constraint. With security blocking turned off, the old constraints still resolve to guzzle 7.8.0 and phpunit 10.5.0.

No functional change: verification

Check	Before	After
composer validate	valid	valid
Fresh composer update (70 packages)	guzzle 7.15.5, phpunit 10.5.65	same 70 package versions
phpunit --exclude-group live (latest dependencies)	OK, 24 tests	OK, 24 tests
composer update --prefer-lowest + unit tests		OK, 24 tests on guzzle 7.15.2 and phpunit 10.5.62
composer audit	no advisories	no advisories

Because a fresh install resolves exactly the same packages as before, and the SDK code is unchanged, the runtime behavior is identical. The only difference in 3.0.1 is the User-Agent version string (3.0.0 → 3.0.1).

The CI matrix (PHP 8.1, 8.2 and 8.3) runs on this PR. Locally I tested on PHP 8.5.