Disable terminal scheduling on non-managed seeds

[Lappihuan]

What this PR does / why we need it:
This disables the terminal on shoots that are not on a managed seed, using the suggestion from #2170 (comment)

Which issue(s) this PR fixes:
Fixes #2170

Special notes for your reviewer:
I'm unsure why there was a check on the garden namespace, but this excluded all shoots in the garden project.
https://github.com/gardener/dashboard/pull/2358/files#diff-6ed2ba0f6d68010cb296caa1dfe39da1cd2acf7dabd973d99fdf670d5849907fL307-R307
This might cause a bug elsewhere.

Release note:

Disable terminal scheduling on non-managed seeds

[gardener-robot]

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[gardener-robot]

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[grolu]

/lgtm

Removing namespace !== 'garden' should not break anything. If a seed is not managed, canLinkToSeed will be false. It was just an assumption that shoot resources in garden namespace will not have a seed that is managed but it is fair to remove it as this could be the case.
Also, using canLinkToSeed as an indicator for the terminal that the seed is managed is considered a workaround. It would be better to actually read the ManagedSeed resources in the backend and add a proper field to the shoot info. But we can target this in a separate PR.

After some internal discussions we found some issues with this PR:
Using the suggested approach, terminals would never be visible for regular users (non operators) as canLinkToSeed would always be false.
Also, for operators it should still be possible to schedule a terminal on the cluster even if the seed cannot be used to schedule a terminal. So disabling those options need to be implemented on the GCreateTerminalSessionDialog. Also, the operator needs to be informed (e.g. via tooltip) why scheduling a terminal on the seed is not possible.
See original comment in issue:

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.

[Lappihuan]

It would be better to actually read the ManagedSeed resources in the backend and add a proper field to the shoot info.

It seems this is the way forward then.

So it depends on #1417

[grolu]

It seems this is the way forward then.
So it depends on #1417

So yes, in the long run this is the preferred solution. @petersutter already started working on watching seeds in the dashboard frontend. However, I cannot give an estimation on the timeline as we are currently blocked with other things.
However, you could still try to build an intermediate solution using canLinkToSeed. The important thing is that you don't disable the button to open the dialog but rather disable the options to schedule a terminal on the seed. This needs to be done on the GCreateTerminalSessionDialog and a tooltip message should be provided informing the operator, why he can only schedule a terminal on the shoot cluster and not on the control plane (seed). Also, the Terminal Runtime needs to be switched to Cluster.
I also noticed a couple of small issues in this context. I will have a look at it.

[Lappihuan]

@grolu is this now superseded by #2373 so i can close this PR?

[grolu]

@grolu is this now superseded by #2373 so i can close this PR?

Yes, I hope that we can merge PR #2373. But there are still some thinks to clarify first :) So I guess we can close this PR.