Drop PHP < 8.2 support (bump minimum to `^8.2`)

Split out from #180 (thanks @JamieSinn for the nudge).

PHP 8.1 reaches security EOL end of December 2025 and 8.0 is already fully EOL. We currently test on ['8.0', '8.1', '8.2'] and most packages declare "php": "^8", i.e. we still support two EOL runtimes.

Per our technical guidelines (Platform Support), we only need to support officially supported (non-EOL) platform versions, and that support should be documented explicitly in implementation tooling (for PHP, the require.php constraint in composer.json). Dropping 8.0/8.1 brings us in line with both.

Proposal: bump the minimum supported PHP to 8.2 across the monorepo.

Changes:

• require.php → ^8.2 in each package's composer.json:
  • hooks/DDTrace, hooks/OpenTelemetry, hooks/Validators (^8 today)
  • providers/CloudBees, providers/Flagd, providers/GoFeatureFlag, providers/Split (^8 today)
  • providers/Flagsmith already ^8.1 <9.0 → align to ^8.2
• CI matrix (.github/workflows/php-ci.yaml): drop 8.0/8.1, add current stable → e.g. ['8.2', '8.3', '8.4']
• Sweep any README/docs stating a PHP floor (guidelines ask for explicit support docs)

Trade-off / caveat:

• Both dropped versions are EOL, so per the guidelines' Removing Support for Platforms we're treating this as non-breaking (regular feat:, minor release; no !). Consumers still on 8.0/8.1 stay on the current released versions.

Update: #179 (in review) already lands part of this:

• drops PHP 8.0 from the CI matrix (now ['8.1', '8.2'])
• bumps providers/Flagd to php: ^8.1 (forced by the google/protobuf runtime dep)
• completes the earlier addendum: testcontainers moved to require-dev, and the on-the-fly composer require removed from dev:test:e2e:setup

Once #179 merges, the remaining scope narrows to:

• drop 8.1 from the matrix → ['8.2', '8.3', '8.4']
• bump require.php to ^8.2 across all packages (Flagd is ^8.1, others still ^8)
• testcontainers → require-dev / drop composer require in e2e setup (done in #179)

Order: land #179 first, then this is a small sweep.

Wait for #179 to land, then inspect the listed packages' composer.json files and .github/workflows/php-ci.yaml. Sweep README and documentation references to the PHP floor. Done means every package declares ^8.2, the CI matrix covers 8.2, 8.3, and 8.4, and no documented support floor contradicts those versions.