Use Symfony 5.4

[saundefined]

Updated the Symfony packages to 5.4,
when we drop PHP 7.4, we can upgrade the packages to 6.3.

@beryllium, what do you think?

[beryllium]

This is great, thanks!

Took me a little bit to get my system ready to test it (and I had to update my "icelus" thumbnail bundle along the way, since 5.4 introduces a change to the dependency injection component.)

BTW, if folks encounter that dependency injection issue:

• In 5.4, the TreeBuilder requires a name as the first argument. For icelus, this name was the name of the root node, icelus.
• The root node variable can now be populated by calling $tree->getRootNode().

In effect, this:

public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder;
        $rootNode = $treeBuilder->root('icelus');

Becomes this:

public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder('icelus');
        $rootNode = $treeBuilder->getRootNode();