Revisit our browserslist config

Closes #29314 (closed).

I kept the Edge patch separate because ATM the update hasn't shipped on all supported Windows versions: https://support.microsoft.com/en-us/help/4541302/the-new-microsoft-edge-is-available (see also my last point)

master branch:

Browsers:
  Chrome for Android: 81
  Firefox for Android: 68
  And_qq: 10.4
  UC for Android: 12.12
  Android: 81
  Baidu: 7.12
  Chrome: 83, 81, 80, 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60
  Edge: 83, 81, 80, 79, 18, 17
  Firefox: 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60
  iOS: 13.4, 13.3, 13.2, 13.0-13.1, 12.2-12.4, 12.0-12.1, 11.3-11.4, 11.0-11.2, 10.3, 10.0-10.2
  Kaios: 2.5
  Opera Mini: all
  Opera Mobile: 46
  Opera: 68
  Safari: 13.1, 13, 12.1, 12, 11.1, 11, 10.1, 10
  Samsung: 11.1

These browsers account for 92.74% of all users globally

This branch:

Browsers:
  Chrome for Android: 81
  Firefox for Android: 68
  And_qq: 10.4
  UC for Android: 12.12
  Android: 81
  Baidu: 7.12
  Chrome: 83, 81, 80, 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60
  Edge: 83, 81
  Firefox: 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60
  iOS: 13.4, 13.3, 13.2, 13.0-13.1, 12.2-12.4, 12.0-12.1, 11.3-11.4, 11.0-11.2, 10.3, 10.0-10.2
  Kaios: 2.5
  Opera Mini: all
  Opera Mobile: 46
  Opera: 68, 67
  Safari: 13.1, 13, 12.1, 12, 11.1, 11, 10.1, 10
  Samsung: 11.1, 10.1

These browsers account for 91.88% of all users globally

This branch without the Edge patch:

C:\Users\xmr\Desktop\bootstrap>npx autoprefixer --info
Browsers:
  Chrome for Android: 81
  Firefox for Android: 68
  And_qq: 10.4
  UC for Android: 12.12
  Android: 81
  Baidu: 7.12
  Chrome: 83, 81, 80, 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60
  Edge: 83, 81, 80, 79, 18, 17
  Firefox: 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60
  iOS: 13.4, 13.3, 13.2, 13.0-13.1, 12.2-12.4, 12.0-12.1, 11.3-11.4, 11.0-11.2, 10.3, 10.0-10.2
  Kaios: 2.5
  Opera Mini: all
  Opera Mobile: 46
  Opera: 68, 67
  Safari: 13.1, 13, 12.1, 12, 11.1, 11, 10.1, 10
  Samsung: 11.1, 10.1

These browsers account for 93.97% of all users globally

Notes:

1. Do note that dropping Opera Mini does not gain us anything, i.e. no prefix is removed/added.
2. We could drop the defaults change, but IMO we should change what we were using because 1 last version isn't proper
3. I'd also argue about the Edge change, but I don't want to hold us back in v5 either. That being said, it does feel too soon to drop it, but depends to when we will actually ship v5.0.0 stable.

Preview: https://deploy-preview-30986--twbs-bootstrap.netlify.app/

EDIT: actually dropping Opera Mini should allow us to use const/let. I will experiment with that later.