Upgraded to composer-runtime-api:^2.1.0

`composer-runtime-api:2.0.x` included deprecations that caused crashes/errors before
`composer/composer:2.0.14`, and a restriction to `"conflict": {"composer/composer": "<2.0.14"}`
did not work ( Ocramius/ProxyManager#700 (comment) ).

In order to clean this up, this fixes #702 by declaring a the dependency to `composer-runtime-api:^2.1.0`
explicitly, effectively excluding `composer/composer:~2.0.0` from installation candiates.

Author: Ocramius

composer.json

@@ -21,11 +21,10 @@
     "require": {
         "php":                       "~7.4.1 || ~8.0.0",
         "laminas/laminas-code":      "^4.3.0",
-        "composer-runtime-api":      "^2.0.0",
+        "composer-runtime-api":      "^2.1.0",
         "webimpress/safe-writer":    "^2.2.0"
     },
     "conflict": {
-        "composer/composer":         "<2.0.14",
         "zendframework/zend-stdlib": "<3.2.1",
         "laminas/laminas-stdlib":    "<3.2.1",
         "doctrine/annotations":      "<1.6.1",