google-mailer | Symfony Google Mailer Bridge | Web Framework library

The problem is not really in monolog-bundle but as follows:

• monolog-bundle requires symfony/http-kernel ~3.4 || ~4.0 || ^5.0
• You restricted all Symfony components to 5.0 (by setting "extra" -> "symfony" -> "require" to 5.0.*). So the only fulfillable requirement for monolog-bundle is http-kernel 5.0.*
• roave/security-advisories works by purposely conflicting with lib versions that have known security issues. In this case, every 5.0.* release has vulnerability CVE-2020-15094 (see https://symfony.com/blog/cve-2020-15094-prevent-rce-when-calling-untrusted-remote-with-cachinghttpclient) and is therefore blocked. So there is no valid version left and Composer aborts.

My suggestion: Symfony 5.0 reached end of life, so use the current release Symfony 5.2.x (Composer constraint "^5.2"). As Symfony uses strict Semantic Versioning there is no disadvantage for going to 5.2 instead of 5.0 (i.e. all code that runs on 5.0 will also run on 5.2).