Upgrade yarn packages to latest

[tacman]

Since this is the demo program that shows best practices, is there any reason not to update to the latest packages? In particular, Webpack and stimulus.

@symfony/stimulus-bridge      2.1.0   -> 3.2.1                                                     
@symfony/webpack-encore       1.4.0 ->  2.1.0                                 
stimulus                      2.0.0  ->  3.0.1  

I'm happy to do a PR, but wanted to see if there was any particular reason to keep these older versions in the branch that works with 6.1.

[javiereguiluz]

There's no reason to keep the old versions, so please submit a PR to update this. Thanks!

[94noni]

Works already done on my PR proposing to add stimulus
#1284
it was an early comment :)

[javiereguiluz]

@94noni sorry for not having taken care of your PR earlier. I've decided to merge this PR because your PR already needs some rebasing, so the new conflicts won't complicate things much.

I'll try to take care of your PR soon. Thanks.

[94noni]

@javiereguiluz great news, yes and as explain I got some conflict/problem trying to recompile new assets before versioning them :s
anyways, nice that this project is up to date now :)