migration symfony 5 4 (#300)

* symfony 5.4 (diff dev)

* symfony 5.4 (working)

* symfony 5.4 (update autoload)

* symfony 5.4 (remove swiftmailer mailer implementation)

* symfony 5.4 (php doc and split Global accessor class)


### Impacted packages:

composer require php:">=7.2.5 <8.0.0" symfony/console:5.4.* symfony/dotenv:5.4.* symfony/framework-bundle:5.4.* symfony/twig-bundle:5.4.* symfony/yaml:5.4.* --update-with-dependencies

composer require symfony/stopwatch:5.4.* symfony/web-profiler-bundle:5.4.* --dev --update-with-dependencies

lib/symfony/polyfill-mbstring/composer.json

@@ -16,7 +16,10 @@
         }
     ],
     "require": {
-        "php": ">=5.3.3"
+        "php": ">=7.1"
+    },
+    "provide": {
+        "ext-mbstring": "*"
     },
     "autoload": {
         "psr-4": { "Symfony\\Polyfill\\Mbstring\\": "" },
@@ -28,7 +31,7 @@
     "minimum-stability": "dev",
     "extra": {
         "branch-alias": {
-            "dev-main": "1.19-dev"
+            "dev-main": "1.26-dev"
         },
         "thanks": {
             "name": "symfony/polyfill",