Gitlab API changing pagination (new keyset pagination system)

Gitlab is changing the API pagination. [Here](https://docs.gitlab.com/ee/api/repositories.html#list-repository-tree) the warning says the following:

> This endpoint is changing to keyset-based pagination. Iterating pages of results with a number (?page=2) [is deprecated](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67509). Support for iterating with a number will become unsupported in GitLab 15.0. Use the new [keyset pagination system](https://docs.gitlab.com/ee/api/index.html#keyset-based-pagination) instead.

It seems we may use that method [here](https://github.com/chaoss/grimoirelab-perceval/blob/fe6f3c944a455c9883f5d40f6e808dc394f7db88/perceval/backends/core/gitlab.py#L480
) , but at least [here](https://github.com/chaoss/grimoirelab-perceval/blob/fe6f3c944a455c9883f5d40f6e808dc394f7db88/perceval/backends/core/gitlab.py#L678) we are using the old one that is going to be removed on Gitlab 15 (to be released on May 22, 2022)

Start in perceval/backends/core/gitlab.py, especially the pagination handling around lines 480 and 678, and compare those requests with GitLab's keyset pagination documentation. Update the affected pagination path so it no longer relies on numbered pages, then verify that repository-tree retrieval still iterates through all results.

Written by the indexing model from the issue text.