Remove default value for agent in policy editor

What does this MR do and why?

Describe in detail what your merge request does and why.

Removes the default value (default) for the kubernetes-agent name in the security policy editor to provide more context that the user must submit the name of the agent. The placeholder does a good job!

Initially I had some confusion on what to provide here and had to lookup the policy schema throughly. The word default makes you think of Kubernetes namespaces rather than an agent name.

Screenshots or screen recordings

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Initially it looks as follows:

the default will be replaced with my-agent-name

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

This I can use some help with....

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