Update alekitto/grcov action

[josecelano]

This action still uses Node 16.

      - id: coverage-grcov
        name: Generate Coverage Report with grcov
        uses: alekitto/grcov@v0.2

Producing a warning:

[Report](https://github.com/torrust/torrust-index/actions/runs/9173014713/job/25220776364)
Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: alekitto/grcov@v0.2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

It's not possible to open an issue on the repo:

https://github.com/alekitto/grcov/commits/master/

And there has not been any activity since May 2023.