🐛 Bug: README.md badges don't have sufficient white-on-green color contrast

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Expected

https://webaim.org/articles/contrast: all text in the README.md should have >= 3.5:1 color contrast between foreground and background text.

Actual

The badges I've been using -which, I should add, use pretty a common default green 😬- only have 2.68:1: https://webaim.org/resources/contrastchecker/?fcolor=FFFFFF&bcolor=21BB42.

Additional Info

The images do apply a shadow of #218334:

...but that's only underneath the text. So my hunch is it really only "counts" halfway.

[JoshuaKGoldberg]

I like #289835:

...but the codecov badge will need to updated with a custom range of colors, I think. Blocked on badges/shields#994 -> badges/shields#9890.

Edit: now, badges/shields#5497 -> badges/shields#9916