Paul doesn't think libcint needs to be pinned and could be any recent version

[joshkamm]

Could start with >=5.3.0

[joshkamm]

Related: #82 — the FetchContent approach on branch issue-82-libcint-4c1e pins to v5.3.0 via GIT_TAG. When we address the version pin, we'll want to update this tag as well. The cache buffer bug fix for 4c1e (described in #82) should ideally target whatever version we upgrade to.