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

scss/_nav.scss

@@ -157,7 +157,7 @@
 }
 
 
-// Justified navigation (flexbox only)
+// Justified navigation
 //
 // Justified nav components are only built for flexbox mode in Bootstrap 4. In
 // previous versions, this component variation was limited to anchor-based nav
@@ -167,13 +167,11 @@
 // Using flexbox, we avoid that problem altogether at the cost of no default
 // justified navigation for default Bootstrap. Sorry, y'all <3.
 
-@if $enable-flex {
-  .nav-justified {
-    display: flex;
+.nav-justified {
+  display: flex;
 
-    .nav-item {
-      flex: 1;
-      text-align: center;
-    }
+  .nav-item {
+    flex: 1;
+    text-align: center;
   }
 }