chore(vue3): remove key inside v-for

What does this MR do?

Vue3 (even in compat mode!) requires :key to be placed not on elements inside v-for but on <template v-for> instead. This is major difference from Vue2 which can't be polyfilled.

How many components are problematic? 1️⃣

So here I'm dropping :key and silencing ESLint for breadcrumb

🤔 How guilty am I? Well, we were using index as key in any case, other two components inside this v-for already had this lint silenced, and since I do not expect breadcrumbs to have even dozens of items I do not see any viable performance penalty here

This is forward-compatible change to support @vue/compat #1981 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.