Investigate why svgs within spans in the icon variant of button don't appear for Safari

## Summary of the bug
#1242 was raised as the close icon didn't display in dialog, however all other close icons displayed. For consistency for the ticket, the svg was moved from out of a span and just added using innerHTML like other components had done. However, there should be some investigation into why the svg within a span didn't appear in safari for icon button on dialog and whether it's a wider issue that svgs within spans don't display.

Note: They seem to appear for icons slotted into regular ic-buttons.

## 🧐 Expected behaviour
Icons should appear as svgs within spans no matter the variant.

Start with issue #1242 and compare the icon-button dialog rendering with regular ic-buttons in Safari, focusing on the span and innerHTML approaches described here. Done means determining whether the problem is specific to the icon variant or affects SVGs within spans more broadly, with the expected icon behavior reproduced.

Written by the indexing model from the issue text.