Require pybind 2.6 for gmxapi Python package extension module.

We currently bundle pybind 2.5 and require only pybind 2.2 for the gmxapi Python package C++ extension module. For robust compatibility with Python 3.9, we should update the pybind requirement to pybind>=2.6 before the 2022 beta release.

Optionally, we could update the way the pybind dependency works, but we may want to tackle that in a separate issue. pybind has updated support for CMake and scikit-build, and has moved toward dependency fulfillment through Python package import rather than bundled git subtrees.

The sample_restraint code remains compatible with GROMACS 2019 and gmxapi >= 0.0.7. We may not need to add a hard requirement there, but might consider updating the compatibility checks. This is probably a separate issue, too.

Related to #3716 (closed) and #3708 (closed).

There are also still references in the repository to issue #2896 (closed). These should be updated.