Guard iOS 14.3 and up to be sandboxed. iOS 14.2 and below has a WebKit Crash

[Brandon-T]

Summary of Changes

• Guard iOS 14.3 and up to be sandboxed.
• iOS 14.2 and below has a crash: "Fatal error: Bug in WebKit: Received neither result or failure.: file WebKit/WebKitSwiftOverlay.swift, line 66"

This pull request fixes #4967

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

It can be tested as a part of manual passes:

• Test session restore
• Test reader-mode
• Test playlist
• Test a bookmarklet like: javascript:alert("hello");

This exact crash we are fixing happens on iOS 14.2 or lower devices. QA most likely does not have such device,
we tested it on our own locally with simulator builds.

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).