Get the Playwright version from the package.json

[elchininet]

This pull request should be merged after #267

To run the end-to-end tests we execute Playwright using a spcific Docker tag. To know the docker tag that we need to use, we read the Playwright Docker image tag from a file placed in the root of the project. This provokes that every time @dependabot creates a pull request updating the @playwright/test package, tests fail because the Playwright Docker image is outdated.

In this pull request, we read the Playwright version from the same package.json and in this way, we can remove the necessity of having the .playwright_docker_version at all.

As part of this pull request the pnpm-lock.yaml file has been deleted because at the moment we use yarn as package manager in the project.