Prevent installation with incompatible symfony/process versions

Sadly, `symfony/symfony` declared `php:>=7.1` compatibility many moons ago: that is generally
technical lack of vision and responsibility on their end, but that ship has sailed.

This means that a `composer update` can and **will** install a dependency graph that is incompatible
with your system's dependencies.

This change prevents installation of `symfony/process` in versions too archaic to actually function.

As a side-effect, `vimeo/psalm` was also upgraded to its latest and greatest.

Ref: https://github.com/vimeo/psalm/releases/tag/4.12.0

composer.json

@@ -29,7 +29,10 @@
         "roave/infection-static-analysis-plugin": "^1.10",
         "roave/security-advisories": "dev-master",
         "squizlabs/php_codesniffer": "^3.6.1",
-        "vimeo/psalm": "^4.11.2"
+        "vimeo/psalm": "^4.12.0"
+    },
+    "conflict": {
+        "symfony/process": "<5.3.7"
     },
     "autoload": {
         "psr-4": {