Fix Public Deploy Keys form to allow expiration dates

What does this MR do and why?

Closes #427842 (closed)

The Public Deploy Keys form has HAML syntax which incorrectly showed a new deploy key form with the expiration date field as read only. This MR fixes the behaviour by:

• Showing an editable version of the input when a new deploy key is being created
• Moving the original read only version of the field to an if? statement. This ensures no expiration date field is presented if an expiration is not set.

Screenshots or screen recordings

Before	After	Description
		Creating a new Deploy Key. The expiration date field is a selectable date, which matches the project deploy keys style.
		Editing an existing Deploy Key
		Expiration date is hidden if no expiry is set

GIF Example

How to set up and validate locally

1. Go to Admin Area > Deploy Keys
2. Create a new Deploy Key. Use a date value like 2024-01-01 for the expiration date. Save the form.
3. Edit the Deploy Key. Confirm the key's expiration date has been saved.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.