Backport Work Item copy changes to legacy issuables

What does this MR do and why?

Backports a few small copy changes from Work Items into legacy issuables:

1. Assign labels -> Select labels
2. Assign to -> Select assignees
3. Delete issue should be text-danger on issue dropdown

This is meant to aid in the eventual transition to work items for issues and MRs; the copy change will happen sooner, meaning one fewer thing the user needs to get accustomed to in the new work item paradigm later.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After	Changed
		danger text on Delete issue
		Assign to -> Select assignees
		Assign labels -> Select labels
		Assign milestone -> Select milestone
		Assign iteration -> Select iteration
		Assign health status -> Select health status
		(MR) Assign to -> Select assignees
		(MR) Assign labels -> Select labels
		(MR) Assign milestone -> Select milestone

How to set up and validate locally

1. Check out this branch
2. Visit an issue
3. Open the actions menu; you'll see the Delete issue item is in text danger (i.e., red)
4. Edit assignees. You'll see that Select assignees is used as the dropdown header.
5. Edit labels. You'll see that Select labels is used as the dropdown header.
6. Visit an MR.
7. Edit assignees. You'll see that Select assignees is used as the dropdown header.
8. Edit labels. You'll see that Select labels is used as the dropdown header.

Related to #450755 (closed), #425467 (closed)