Drop all support for IE browsers (#767)

Microsoft [no longer supports](https://blogs.windows.com/windowsexperience/2022/06/15/internet-explorer-11-has-retired-and-is-officially-out-of-support-what-you-need-to-know/)
any version of IE, so IE is no longer supported by this library.

.changeset/browser-support-revision.md

@@ -0,0 +1,5 @@
+---
+'focus-trap-react': major
+---
+
+- Revised and clarified official browser support (still as broad and deep as _reasonably_ possible).

.changeset/drop-ie.md

@@ -0,0 +1,6 @@
+---
+'focus-trap-react': major
+---
+
+🚨 __Breaking:__ Dropped support of IE browsers, all versions.
+    - IE11 was [officially retired](https://blogs.windows.com/windowsexperience/2022/06/15/internet-explorer-11-has-retired-and-is-officially-out-of-support-what-you-need-to-know/) on June 15, 2022 (6 weeks ago). There are no longer any versions of IE that are still maintained or even supported by Microsoft.

README.md

@@ -33,13 +33,21 @@ npm install focus-trap-react
 
 ### React dependency
 
-React `>= 16.0.0`.
+React `>= 16.3.0`
 
 ## Browser Support
 
-Basically IE9+.
+As old and as broad as _reasonably_ possible, excluding browsers that are out of support or have nearly no user base.
 
-Why? Because this module's core functionality comes from focus-trap, which uses [a couple of IE9+ functions](https://github.com/davidtheclark/tabbable#browser-support).
+Focused on desktop browsers, particularly Chrome, Edge, FireFox, Safari, and Opera.
+
+Focus-trap-react is not officially tested on any mobile browsers or devices.
+
+> ⚠️ Microsoft [no longer supports](https://blogs.windows.com/windowsexperience/2022/06/15/internet-explorer-11-has-retired-and-is-officially-out-of-support-what-you-need-to-know/) any version of IE, so IE is no longer supported by this library.
+
+> 💬 Focus-trap-react relies on focus-trap so its browser support is at least [what focus-trap supports](https://github.com/focus-trap/focus-trap#browser-support).
+
+> 💬 Keep in mind that performance optimization and old browser support are often at odds, so tabbable may not always be able to use the most optimal (typically modern) APIs in all cases.
 
 ## Usage