Mysteriously missing math glyphs

I noticed that 3 math glyphs that I need were missing from Source Code Pro, namely

```
U+222A ∪ union
U+2208 ∈ element
U+220B ∋ suchthat
```

Interestingly these glyphs were added/proposed in pull request #213 together with many others, where @pauldhunt [said](https://github.com/adobe-fonts/source-code-pro/pull/213#issuecomment-842792389) they'd redraw the characters for a new size and include them in the next release. A new release has happened this year (2023-04-12) but the glyphs were unfortunately not included.

I would like to reopen the original request of #213 to add commonly used math glyphs. This issue may be related to the vague request in issue #294 for "more math symbols".