Deprecate IE conditional comments support

Problem/Motivation

Coming from fixing the CSS Variables po[ln]yfill for IE, i stumbled upon this:

How to reproduce:

* Want to add library CSS/JS browser-conditional like so:

my_library:
  js:
    run-cssvars-polyfill.js:
      browsers: { '!IE': false }

Result:
* CSS/JS is wrapped in a browser-switch comment like this:

<!--[if IE]>
<script src="/modules/contrib/cssvars/polyfill/libraries/css-vars-polyfill.min.js?q16qed"></script>
<![endif]-->

Alas, M$ dropped the conditional script comment tag in 2016 since IE10 (SO). So it seems that the resulting browser switch comments only work up to IE9 (see #2390621: [policy, no patch] Update Drupal's browser support policy on why this is EOL but still supported).

Proposed resolution

* Update docs.

Once we drop IE9,

* Remove the functionality.
* Error out or add warning if developers use it.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet