If you ran into the

Your requirements could not be resolved to an installable set of packages.

error message while running composer update today, see this issue:
https://www.drupal.org/project/drupal/issues/3599842

In brief, quoting the the issue:

While these were fixed a few weeks ago (see https://github.com/advisories/GHSA-hq7v-mx3g-29hw), we can see the composer datasource was only registered with the vulnerabilities on June 11th.

and

You can temporarily fetch the 2.11.0 version of guzzlehttp/psr and alias it to 2.8.1 with this:
composer require "guzzlehttp/psr7:2.11.0 as 2.8.1"
Revert this once a new Drupal core is released by deleting the guzzlehttp/psr7": "2.11.0 as 2.8.1", line in composer.json, and then updating Drupal core.