Make copy for entering URLs consistent for issues and epics

What does this MR do and why?

Make copy for entering URLs consistent for issues and epics:

• Fields where you can enter an issue, epic or MR URL should use the phrase: Enter <item> URL instead of Paste <item> link to follow:
  • Style guidance for enter: https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#enter
  • Our own terminology: we use "link" in "linked items", so URL is more precise.
• Where the placeholder text refers to #123 and !123, these should be called <#issue ID> and <!merge request ID>, with ID capitalized.

Related to this thread: #427975[Create_add_workflows.png] (comment 1920852406)

Follow-up MR will handle work items, such as tasks or OKRs.

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

Issues

Add existing task as child item

Before	After
	no change

Linked items:

Before	After

Epics

Add existing child issue:

Before	After

Add existing child epic:

Before	After

Linked epics:

Before	After

Incidents

Linked incidents or issues:

Before	After

How to set up and validate locally

1. Check out the branch locally.
2. In GDK, go to an issue, incident, or epic.
3. From Child... or Linked... widgets, select options to add an existing item.