Drop support for composer v1

All the PHP code should still be compatible with [email protected]. So why?

* We need to swap-out an abandoned dependency (`togos/gitignore` =>
  `civicrm/gitignore`) to address compatibility w/php82.

* But packagist.org refuses to serve metadata for new dependencies
  (like `civicrm/gitignore`) to [email protected] clients... regardless of
  substantive compatibility.

* So it's a metadata problem (not a code problem). But it still affects
  operations... if [email protected] cannot download then code, then it won't
  work!

* Effectively, it's strict XOR: run cleanly on php82... XOR... keep
  compatibility with [email protected]. Between those, php82 seems more
  important.

.github/workflows/main.yml

@@ -12,7 +12,7 @@ jobs:
       matrix:
         php: [7.2, 7.3, 7.4, 8.0, 8.1]
         os: [ubuntu-latest]
-        composer: [v1, v2]
+        composer: [v2]
     name: ${{ matrix.php }} - PHPUnit - Composer ${{ matrix.composer }}
     steps:
       - uses: actions/checkout@v2

composer.json

@@ -14,12 +14,12 @@
         }
     ],
     "require": {
-        "composer-plugin-api": "^1.1 || ^2.0",
+        "composer-plugin-api": "^2.0",
         "php": ">=7.2",
         "civicrm/gitignore": "~1.2.0"
     },
     "require-dev": {
-        "composer/composer": "~1.0 || ~2.0",
+        "composer/composer": "~2.0",
         "phpunit/phpunit": "^8.5 || ^9.5",
         "friendsofphp/php-cs-fixer": "^2.3",
         "totten/process-helper": "^1.0.1"