fix(ci): update PHP version from 8.1 to 8.3 in CI matrix and composer…

[katag9k]

Description

The wp-test-runner Docker image no longer supports PHP 8.1, causing all 12 PHP 8.1 CI jobs to fail with "Unsupported PHP version: 8.1" before any tests run.

This PR updates:

• .github/workflows/unit-tests.yml — Replaces PHP 8.1 matrix entries with 8.3
• composer.json — Updates require constraint and platform config to PHP 8.2 (matching CI minimum), and bumps local test scripts to PHP 8.3
• composer.lock — Regenerated after platform change

Changelog Description

Fixed

• Fixed CI test failures by replacing unsupported PHP 8.1 with PHP 8.3 in the test matrix and updating the minimum platform requirement to PHP 8.2.

Pre-review checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally or in Codespaces (or has an appropriate fallback).
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).
• I've created a changelog description that aligns with the provided examples.

Pre-deploy checklist

• VIP staff: Ensure any alerts added/updated conform to internal standards (see internal documentation).
  No alerts were added or updated — N/A.

Steps to Test

1. Check out PR.
2. Verify CI jobs pass — PHP 8.2, 8.3, and 8.4 matrix entries should all run successfully.
3. Confirm no PHP 8.1 jobs remain in the workflow run.