…upgrade (elastic#119745)

The deprecations for ML snapshots that are too old and need
upgrading are supposed to have a "Fix" button next to them.
Unfortunately this disappeared when a message was reworded
in elastic/elasticsearch#79387.

This change adjusts the regex that the upgrade assistant
uses to detect which deprecations to add the "Fix" button
to so that it will match both "Model snapshot" and "model
snapshot". The test data is also updated to match the new
backend text.

This change is designed to work with elastic/elasticsearch#81060.