Iris 3.5 libnetcdf pin?

[trexfeathers]

libnetcdf v4.9.1 has caused performance regressions (#5182), and produced many new low-level warnings of minimal consequence (#5187). Our two corrective options:

• Investigate fixes before the release - complicated, time-consuming, need to sacrifice other 3.5 work
• Pin libnetcdf <4.9 - pins are unsustainable long term and create awkwardness for users creating environments.

[bjlittle]

@trexfeathers If we vote to temporarily pin we should raise an issue to ensure that we unpin again 👍