Avoid early rendering of links in progress bar [#3539629]

Problem/Motivation

I noticed that the links generated in FormsStepsProgressBarBlock are rendered into markup instead of just returning render arrays. As a result, the title and URL of individual items can not be accessed or modified in hooks or the item-list template.

Proposed resolution

Since early rendering is generally discouraged I would suggest creating link render arrays for each item.

Issue fork forms_steps-3539629

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3539629-avoid-early-rendering changes, plain diff MR !22
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