seldon-core-operator should use version 1.15 for `executor-image` in `track/1.15`

[orfeas-k]

Charm is using version 1.14.0 instead of 1.15 for no apparent reason. We 've probably missed this during upgrading from 1.14 to 1.15.

[i-chvets]

This issue is related/duplicates #200

[orfeas-k]

@i-chvets They indeed relate but this issue is track/1.15-specific which is used in CKF 1.7, not 1.8.

[orfeas-k]

I think we should remove Kubeflow 1.8 label