Update composer/installers in root composer.json and lock so we can use Composer 2

Problem/Motivation

Running composer install on Drupal 8.9 with composer 2 fails. #3126566: Allow Drupal to work with Composer 2 didn't get finished for D8 because of all the fun we had with #3134648: [backport, needs scheduling] Don't pin the composer/installers version in drupal/core-recommended.

You get the following error:

composer install                                                                                                                                                                                                                              743ms  Fri 17 Jul 00:03:04 2020
The "composer/installers" plugin was skipped because it requires a Plugin API version ("^1.0") that does not match your Composer installation ("2.0.0"). You may need to run composer update with the "--no-plugins" option.
> Drupal\Composer\Composer::ensureComposerVersion
Installing dependencies from lock file (including require-dev)
Verifying lock file contents can be installed on current platform.
Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - composer/installers is locked to version v1.7.0 and an update of this package was not requested.
    - composer/installers v1.7.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.

Use the option --with-all-dependencies to allow upgrades, downgrades and removals for packages currently locked to specific versions.

You are using a snapshot build of Composer 2, which some of your plugins seem to be incompatible with. Make sure you update your plugins or report an issue to them to ask them to support Composer 2. To work around this you can run Composer with --ignore-platform-req=composer-plugin-api, but this may result in broken plugins and bigger problems down the line.

Proposed resolution

Update composer/installers to a version that support Composer2. Now we're not pinning this only affect core development.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

composer/installers updated to 1.9.0 (same as D9).