[policy, no patch] Update Drupal's browser support policy

https://www.drupal.org/docs/8/system-requirements/browser-requirements lists some extremely outdated browsers, such as Firefox 5 and Safari 5. It's unlikely that any core developers or testers test on these browsers. The browser vendors have long ago abandoned security support for them, so for a core developer or tester to even install one of them potentially opens their computer up to a security vulnerability.
In Drupal 8.4, we updated to jQuery 3: #2533498: Update jQuery to version 3. https://jquery.com/upgrade-guide/3.0/#browser-support lists what is officially supported by that, which is e.g., only Current and (Current - 1) of Firefox and Safari.

Update https://www.drupal.org/docs/8/system-requirements/browser-requirements to:
- The latest release of each of the latest two supported major versions of:
  - Desktop browsers:
  - Mobile browsers:
- The latest supported release of the latest major version of:
  - Desktop browsers:
  - Mobile browsers:
Clarify what our browser support means.
- Drupal accepts bug reports for supported browsers.
- Contributors manually test significant changes in multiple browsers to watch for regressions, but this is somewhat based on the contributors available and the particular issue. We do not yet have full automated browser testing for our supported browsers.
- By default, Drupal core does not accept bug fixes to unsupported browsers. Committers and the security team will assess the risk on a case-by-case basis and make exceptions to this rule when needed.
- Drupal can start using any given browser feature once all the supported browsers have either native support or a polyfill. In each major release, polyfills that are no longer needed (because all supported browsers have added the feature natively) will be removed.

Drupal core's browser support has been updated, see https://www.drupal.org/docs/8/system-requirements/browser-requirements for the current policy.