[6.x] Even out fieldtype icon stroke widths

[jaygeorge]

Description of the Problem

Closes #15652

Most fieldtype icons draw a 1px stroke in a 14×14 viewBox. Five render thinner, because they're either scaled down inside a group or drawn in a wider viewBox with no adjustment to the stroke. Textarea stands out the most.

What this PR Does

Bumps the stroke-width on each affected icon to make up for the scaling, so they render at 1px like the rest:

• Textarea and List, which are scaled down inside a group
• Collections, Taxonomy (used by Taxonomies and Terms) and User Groups, which use a wider viewBox

I went with the stroke width rather than redrawing the paths into a 14×14 viewBox. It's one attribute per file, the shapes stay exactly as they are, and the diff is easy to check. Redrawing would mean regenerating every path and checking the shapes still match, all to get the same rendered result.

How to Reproduce

1. Open any blueprint and click Add Field → Create Field.
2. Compare Textarea, List, Collections, Taxonomies, Terms and User Groups with Markdown, Entries or Users.