Upgrade to macOS 11 runner image for CI

[eliperkins]

Depends on:

• 111

Related to:

• https://github.com/cashapp/AccessibilitySnapshot/issues/98

Closes #98

This requires migrating

• from Xcode 11.3.1 to Xcode 11.7
• from iOS 13.3 to iOS 13.7
• from Xcode 12.2 to Xcode 12.5.1
• from iOS 14.2 to iOS 14.4

[eliperkins]

@NickEntin I believe this is ready to go now! Wanna take a look?

[NickEntin]

This looks great Eli, thanks for taking care of this migration!