fix: downgrade luxon version

* The Invenio ecosystem is still using v1.x of Luxon (see
CERNDocumentServer/cds-rdm#209 (comment)).
The new peer dependency on v3.x of Luxon is causing dependency conflicts
that result in a failed build.

* This commit TEMPORARILY downgrades luxon to v1.x while we find a way
to upgrade the rest of the Invenio modules, so that we're unblocked
building new versions.

* None of the code in this repository appears to be affected by the
major changes between v1 and v3:
https://moment.github.io/luxon/#/upgrading

Author: palkerecsenyi