Fix for R < 4.3.0

[damonbayer]

I was unable to install feasts with R < 4.3.0, with the following error:

object 'chooseOpsMethod' not found whilst loading namespace 'feasts'

This is because chooseOpsMethod was introduced in R 4.3.0.

My solution is to only export chooseOpsMethod.gg_tsensemble when R >= 4.3.0 is used.

Evidence of my fork succeeding and the current main failing: https://github.com/damonbayer/bookworm_feasts/actions/runs/17652535292.