[3.x] Compatibility with Laravel 9 (#802)

* Test on Laravel 9

* Don't extend final Kernel class

* Make FilesystemTenancyBootstrapper compatible with Flysystem v3

Co-authored-by: George <jiri.zizka@funfirst.cz>

* Update tenant maintenance mode te be in line with Laravel

* Exclude PHP 7.4 <> L9 combination from testing

* add root_override-related assertions

* getPrefix -> getPathPrefix

* handle / inconsistency in s3 prefix

* Refactor Storage facade changes

Co-authored-by: George <jiri.zizka@funfirst.cz>
Co-authored-by: Samuel Štancl <samuel.stancl@gmail.com>

.github/workflows/ci.yml

@@ -16,7 +16,10 @@ jobs:
     strategy:
       matrix:
         php: ["7.4", "8.0"]
-        laravel: ["^6.0", "^8.0"]
+        laravel: ["^6.0", "^8.0", "^9.0"]
+        exclude:
+          - laravel: "^9.0"
+            php: "7.4"
 
     steps:
     - uses: actions/checkout@v2