Remove Jinja2 version pinning
It was introduced in response to https://github.com/datalad-handbook/book/issues/821,
but the underlying issue seems resolved (no import error during linkcheck).
An upgrade is necessary because of a vulnerability report in Jinja2, which was
patched in version 3.1.3.
Vulnerability report: https://github.com/datalad-handbook/book/security/dependabot/1
