Migrate coverage reporting from Coveralls to Codecov

[Copilot]

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

Changes

• .github/workflows/ci.yml: Simplifies the coverage step (drops the erroneous mkdir -p mkdir -p build/logs, outputs clover.xml directly), replaces Coveralls upload with codecov/codecov-action@v5 using CODECOV_TOKEN.
• README.md: Swaps Coveralls badge for Codecov badge.
• composer.json: Removes php-coveralls/php-coveralls from require-dev.

Original prompt

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

Changes required

1. .github/workflows/ci.yml

In the phpunit job:

• In the "Run PHPUnit with coverage" step, change the clover output path from build/logs/clover.xml to clover.xml (and remove the mkdir -p build/logs line if present).

• Replace the "Upload coverage report to Coveralls" step 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:

[![Coverage](https://codecov.io/github/brick/date-time-doctrine/graph/badge.svg)](https://codecov.io/github/brick/date-time-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 ☂️