Use known ranges when defaulting to a PHP version

This is useful e.g. for testing branches in parallel, or when running against custom repositories that might receive unexpected updates.

bin/compile

@@ -363,9 +363,9 @@ composer validate --no-plugins --no-check-publish --no-check-all --quiet "$COMPO
 status "Preparing platform package installation..."
 
 if [[ $STACK == "heroku-20" ]]; then
-	HEROKU_PHP_DEFAULT_RUNTIME_VERSION="^7.3.0 | ^8.0.0"
+	HEROKU_PHP_DEFAULT_RUNTIME_VERSION="^7.3.0 | ^8.0.0 <8.3"
 else
-	HEROKU_PHP_DEFAULT_RUNTIME_VERSION="^8.0.0"
+	HEROKU_PHP_DEFAULT_RUNTIME_VERSION="^8.0.0 <8.3"
 fi
 export HEROKU_PHP_DEFAULT_RUNTIME_VERSION
 # extract requirements from composer.lock

bin/util/platform.php

@@ -205,7 +205,7 @@ function mkmetas($package, array &$metapaks, &$have_runtime_req = false) {
 		// we can't compute a resulting version rule (that's the whole point of the custom installer that uses Composer's solver), so throwing an error is the best thing we can do here
 		exit(3);
 	}
-	file_put_contents("php://stderr", "\033[1;33mNOTICE:\033[0m No runtime required in $COMPOSER_LOCK; using PHP ". ($require["heroku-sys/php"] = getenv("HEROKU_PHP_DEFAULT_RUNTIME_VERSION") ?: "^7.0.0") . "\n");
+	file_put_contents("php://stderr", "\033[1;33mNOTICE:\033[0m No runtime required in $COMPOSER_LOCK; using PHP ". ($require["heroku-sys/php"] = getenv("HEROKU_PHP_DEFAULT_RUNTIME_VERSION") ?: "*") . "\n");
 } elseif(!isset($root["require"]["php"])) {
 	file_put_contents("php://stderr", "\033[1;33mNOTICE:\033[0m No runtime required in $COMPOSER; requirements\nfrom dependencies in $COMPOSER_LOCK will be used for selection\n");
 }