GH Actions: version update for ramsey/composer-install

The action used to install Composer packages and handle the caching has released a new major (and some follow-up patch releases), which means, the action reference needs to be updated to benefit from it.

Refs:
* https://github.com/ramsey/composer-install/releases/tag/2.0.0
* https://github.com/ramsey/composer-install/releases/tag/2.0.1
* https://github.com/ramsey/composer-install/releases/tag/2.0.2

.github/workflows/continuous-integration.yml

@@ -28,7 +28,7 @@ jobs:
           php-version: "${{ matrix.php-version }}"
           ini-values: error_reporting=-1, display_errors=On
           coverage: "none"
-      - uses: "ramsey/composer-install@v1"
+      - uses: "ramsey/composer-install@v2"
       - name: "Run the linter"
         run: "composer lint -- --colors"
 
@@ -42,6 +42,6 @@ jobs:
           php-version: "7.4"
           tools: "phpstan:0.12.99"
           coverage: "none"
-      - uses: "ramsey/composer-install@v1"
+      - uses: "ramsey/composer-install@v2"
       - name: "Run PHPStan"
         run: "phpstan analyse -c phpstan.neon -l 4 getid3"