Drop support for outdated browser versions for integrating SDK

Background / User story

Before we can release a version of Adblock Plus that uses the SDK (see #972 (closed)), we need to make sure that we drop support for browser versions that the SDK doesn't support.

What to change

• Design: N/A
• Research: https://www.notion.so/adblock/Which-browser-versions-do-we-need-to-support-with-our-web-extensions-5f13959526f342e49e22446ab5fa21fe (internal)
• Spec: N/A
• Legal: N/A
• Development:
  • Update minimum supported browser versions to:
    • Chrome 77
    • Firefox 63
    • Microsoft Edge 79 (unchanged)
    • Opera 64

Hints for testers

This change also removes some polyfills that were explicitly mentioned as relating to browser versions that we no longer support. Therefore we should verify that the oldest supported browsers still work as expected:

• browser.i18n.getUILanguage() returns a locale string that uses - instead of _, for separating the two locale components (e.g. used for locale-specific documentation links).
• No errors are thrown in the background page and request blocking works as expected (incl. cannot block top-level frame).
• Custom elements in the UI should not break (e.g. language selection dropdown, custom tooltips, custom filter table).

It also updates the Chromium and Firefox versions that are being used for running the "webext" child pipeline's end-to-end tests on the oldest supported browsers.

Hints for translators

N/A

Further information

This will enable us to use the following web technologies:

• window.customElements
• import/export statements (except export * as and in service worker)