System labels should not be displayed when editing labels for an execution

[fhussonnois]

Describe the issue

Currently, when editing labels for an execution system labels are displayed.

We should hide them because system labels cannot be removed/edited/added by a user.

A user could be tempted to modify them, which would lead to an error and degrade the user experience;

Environment

• Kestra Version: develop

[MilosPaunovic]

This was handled by @loicmathieu few days ago, just waiting for him to confirm and close if alright now.

[loicmathieu]

There is no more issue as the backend check that the system labels passed by the frontend are the same that the existing one.
But here, Florian said that it would be better to avoid displaying them at all.

As it's not a big deal, let's move this in 0.21, @anna-geller or @Ben8t could then decided if it needs to be done or if it's OK like this.