bug #436 [Maintenance] Remove php platform from composer (lchrusciel)

This PR was merged into the 1.7 branch.

Discussion
----------

I've removed the platform from the composer to let flex detect the current environment based on used CLI PHP. In addition, I've added composer v2

Replacecment for #434 

Commits
-------

5b69107 [Maintenance] Remove php platform from composer
aaa9670 [Maintenance] Update composer (on PHP 7.3)
9fdd1d3 [Maintenance] Switch to composer v2
e48fcea [Migration] Add nullable for product price

.travis.yml

@@ -39,7 +39,7 @@ jobs:
 
                 - echo "memory_limit=4096M" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
 
-                - composer self-update --1
+                - composer self-update --preview
 
             install:
                 - if [ ! -z "${SYMFONY_VERSION}" ]; then bin/require-symfony-version composer.json "${SYMFONY_VERSION}"; fi

composer.json

@@ -82,9 +82,6 @@
         ]
     },
     "config": {
-        "platform": {
-            "php": "7.3.15"
-        },
         "preferred-install": {
             "*": "dist"
         },