Improve feature flag restart terminology

[nwmac]

Summary

Fixes #11305

Occurred changes and/or fixed issues

This PR renames the column name on the feature flag page and also the message in the confirmation modal.

The terminology now is 'Restart Rancher' (where 'Rancher' is the vendor name)

Technical notes summary

Table columns only accept a label key which can not have params, so we evaluate the table header with the vendor name when we add the table headers for the feature flag table.

Areas or cases that should be tested

1. Visually check that the column name has changed.
2. Try and activate/deactivate a feature flag that requires restart and check the message is correctly updated.

Screenshot/Video

List, showing new name for list column for restart:

Modal dialog for activate/de-activate:

Checklist

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[aalves08]

LGTM

[aalves08]

LGTM