Highlight expired SSH or PAT credentials in the credential inventory

What does this MR do?

This MR updates the credentials inventory to display the expiry date (if set) for PATs (personal access tokens) and SSH keys #214809 (closed).

The expiry dates are shown in one of four states:

1. Doesn't exist (Never)
2. Exists and hasn't expired (normal colour and no icon)
3. Exists but is expiring soon (orange colour and a warning icon, soon is defined in app/models/concerns/expirable.rb:20)
4. Exists but has expired (red colour and an error icon)

What changes are made:

1. Moved expiry column contents to a shared HAML template
2. Added expiry column to SSH keys table
3. Added to the credentials inventory shared examples to include expiry date tests
4. Updated existing credentials inventory specs to use the new shared examples

Screenshots

	Smaller devices	Larger devices
SSH keys
PATs

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Relates #214809 (closed)