When does Iris' mesh support stop being in iris.experimental?

[trexfeathers]

Points to consider:

• Freedom to make breaking changes
• Increasing uptake of the mesh features - we're starting to learn more about what people need
• Users will engage differently with something marked as experimental

[trexfeathers]

Starter for 10: definitely after 3.9 (#5570)

[trexfeathers]

See #5990