Remove doctrine-specification package

[acelaya]

The package has served the project well, and it was important to make API restrictions a reality.

However, it is mostly stale. They haven't released a new version, created an issue or committed in more than 2 years.

With the upcoming Symfony 7 and Doctrine ORM 3, this package is going to become a blocker to update.

Removing it seems the most sensible and future-proof option, but it's not going to be an easy task.

API and database tests will come handy here.

[acelaya]

Short term, and to avoid blocking updates to latest Symfony and Dcotrine versions, it would be ok to fork the doctrine-specification repo, update dependencies, check CI passes, then republish as shlinkio/doctrine-specification.

That would give time to migrate away from the library progressively.

[acelaya]

For now, the package has been forked in https://github.com/shlinkio/doctrine-Specification, adding support for Doctrine ORM 3.0, Symfony 7.0 and PHP 8.2/83