Migrate coverage reporting from Coveralls to Codecov

[Copilot]

Replaces Coveralls with Codecov for coverage reporting, mirroring the migration done in brick/math.

Changes

• .github/workflows/ci.yml: In all three phpunit jobs (mysql, mariadb, postgres):
  • Fix broken mkdir -p mkdir -p build/logs command; output clover directly to clover.xml
  • Replace php-coveralls upload step with codecov/codecov-action@v5
• README.md: Swap Coveralls badge for Codecov badge
• composer.json: Drop php-coveralls/php-coveralls from require-dev

Original prompt

Migrate from Coveralls to Codecov for coverage reporting in brick/geo-doctrine, mirroring the changes made in brick/math#109.

Changes required:

1. .github/workflows/ci.yml: In the phpunit job, simplify the "Run PHPUnit with coverage" step (remove mkdir -p build/logs, change clover path from build/logs/clover.xml to clover.xml). Replace the "Upload coverage report to Coveralls" step (which runs vendor/bin/php-coveralls with COVERALLS_REPO_TOKEN) with:

- name: Upload coverage to Codecov
  uses: codecov/codecov-action@v5
  env:
    CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
  if: ${{ matrix.php-version == env.COVERAGE_PHP_VERSION }}

2. README.md: Replace the Coveralls badge with a Codecov badge: [![Coverage](https://codecov.io/github/brick/geo-doctrine/graph/badge.svg)](https://codecov.io/github/brick/geo-doctrine)

3. composer.json: Remove php-coveralls/php-coveralls from require-dev (if present).

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[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️