Thanks @eris!
As reference for others: I fixed it by uninstalling and reinstalling like so:
Uninstall composer:
sudo apt remove --purge composer
Download the installer script using PHP:
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
Get the latest Composer installer hash and verify the installer script:
HASH="$(curl -sS https://composer.github.io/installer.sig)"
php -r "if (hash_file('SHA384', 'composer-setup.php') === '$HASH') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
Run the installer:
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer
Remove the installer:
php -r "unlink('composer-setup.php');"
Verify installation:
composer --version
Update the project dependencies to their latest versions based on the constraints defined in the composer.json file:
composer update
Update Composer itself to the latest version:
composer self-update