[policy, no patch] Drop IE9 and IE10 support from Drupal 8.3

Split-off from #2390621: [policy, no patch] Align Drupal 8.3's browser support with jQuery 3's. Decide on exceptions.

Problem/Motivation

• Supporting old versions of IE is hard and costly. See, for example, #2339491: Ajax requests on forms with files fail on IE 9.
• For Drupal 8.3, we would like to update jQuery UI to 1.12, but jQuery UI has dropped official support for IE9 and IE10. See #2809427: Update jQuery UI to 1.12 and https://jqueryui.com/browser-support/.
• Drupal 8.3 is scheduled to be released on April 5, 2017. Microsoft is dropping support of IE9 and IE10 on April 11, 2017. See #2390621-35: [policy, no patch] Align Drupal 8.3's browser support with jQuery 3's. Decide on exceptions.. Site owners who don't want to drop legacy IE support before Microsoft does can wait a week before updating to Drupal 8.3.

On the other hand, counter-arguments to above:

• Other projects, including advanced JavaScript ones, still support IE9. E.g., the latest versions of jQuery and Ember still do.
• jQuery UI 1.12 still works on IE9 and IE10 despite not officially supporting them.
• IE8 continued to have a large number of users for months/years after Microsoft discontinued support. IE9 might too.

Related: #1030090: Document our browser support list

Proposed resolution

1. Allow #2809427: Update jQuery UI to 1.12 into Drupal 8.3.
2. Decide that as of Drupal 8.3, we will no longer accept non-security bug reports that only affect IE10-. Policy for security issues needs a separate issue. Pending that issue, see #2390621-21: [policy, no patch] Align Drupal 8.3's browser support with jQuery 3's. Decide on exceptions..
3. Update https://www.drupal.org/docs/8/system-requirements/browser-requirements to set IE11 instead of IE9+ as what is known to work well with Drupal 8.
4. Send out an announcement somewhere ahead of 8.3's release?

Alternatively, decide to do #1 above, but postpone the rest until 8.4 or later, based on when jQuery drops IE9 support, jQuery UI stops working entirely on IE9, or some other compelling technical reason requires us to drop it.

Remaining tasks

Per #2390621-37: [policy, no patch] Align Drupal 8.3's browser support with jQuery 3's. Decide on exceptions., the Drupal 8 core committers think it's reasonable to do all of the proposed resolution (not just #1). However, leaving this issue open for further comments before making that official.