WebVTT with timestamp tags no longer parses them #692

Describe the bug
Loading a video with WebVTT subtitles that include timestamp tags e.g.

WEBVTT

00:00:00.030 --> 00:00:02.119
the<00:00:00.539> art<00:00:00.750> of<00:00:00.870> digital<00:00:01.260> preservation<00:00:01.469> by

... will no longer parse those tags, they will end up in the transcript as text.

This is because DOMPurify sanitizes them so by the time the content reaches parseWebVTT, the chevrons have become &lt; / &gt;, and they're not parsed as tags.

Version tested
Main (4.7)

To Reproduce
Use a track file similar to the one pasted above as subtitles and view the transcript.

Expected behavior
Timestamp tags parsed, at least as far as ignoring them (as happened in previous versions)

Screenshots

4.7

4.6

This is in our page that uses Able but it is just a transcript div with some CSS.