Phase out LaCrOS enum value

[jrobbins]

The enterprise team requested that the LaCrOS platform option should no longer be offered when rolling out features, but it should still remain for past rollouts.

We cannot simply remove the LaCrOS option, because it is still needed for old rollouts to display a string rather than an int or nothing.

In this PR:

• Introduce the concept of an obsolete enum value, and mark LaCrOS as obsolete.
• When constructing sl-select options, don't include obsolete options unless they are part of the current field value.