global: check if we can depend on elasticsearch7 instead elasticsearch PyPI module.

[lnielsen]

It would be an advantage if we're able to install invenio-search with support for all ES/Opensearch versions at the same time.

Currently, you have to install invenio-search[elasticsearch7] or invenio-search[elasticsearch6] which will install either elasticsearch in version v6.x or v7.x (i.e. we cannot install both versions at the same time.

If instead we could rely on the elasticsearch6 and elasticsearch7 packages we would be able to have both installed at the same time, and we could avoid conditional dependencies.