Flexbox all the time (Drop IE9 support and remove $enable-flex option) (#21389)

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

1 files changed, 1 insertions, 1 deletions

diff --git a/grunt/postcss.js b/grunt/postcss.js
index 7e7eb4a7e..a2ea78f5b 100644
--- a/grunt/postcss.js
+++ b/grunt/postcss.js
@@ -29,7 +29,7 @@ module.exports = {
       // For example, at the time of writing, Edge 20 on an up-to-date system uses EdgeHTML 12.
       // See also https://github.com/Fyrd/caniuse/issues/1928
       'Edge >= 12',
-      'Explorer >= 9',
+      'Explorer >= 10',
       // Out of leniency, we prefix these 1 version further back than the official policy.
       'iOS >= 8',
       'Safari >= 8',