Undo poetry version pinning

[dfsp-spirit]

We pinned the poetry version to 1.4.0 in some places due to #451, this can be undone with the release of pydata-sphinx-theme v0.13.3, which fixes their issue 1253 that was the source of our problems.

We should undo the version pinning in:

• our GitHub workflow files
• our .gitlab-ci.yml file for the ESI runners

One thing to keep in mind when updating the version of pydata-sphinx-theme to >=0.13.3 is that we also need to update the version of sphinx-book-theme to >=1.0.1, as discussed in pydata-sphinx-theme issue 1274.