Disable terminal scheduling on non-managed seeds in the dashboard

[petersutter]

What would you like to be added:
The dashboard should disable or hide the option to schedule a terminal on a Seed if it is not a managed seed. The frontend should detect whether a Seed is managed and adjust the available options accordingly.

Why is this needed:
Currently, users may encounter the error "terminal cannot be hosted on non-managed seed" (introduced with #2169) when trying to schedule a terminal on an unmanaged Seed. Disabling or hiding this option upfront would prevent this error and improve the user experience.

[petersutter]

The canLinkToSeed indicator could be used to determine if a seed is managed, but this should be considered a workaround.

dashboard/backend/lib/services/shoots.js

Line 338 in ec8b341

data.canLinkToSeed = !!(await client['core.gardener.cloud'].shoots.get('garden', seed.metadata.name))