Insecure HTTP client not happy with a self-signed certificate

This issue has been migrated from [#11437](https://github.com/matrix-org/synapse/issues/11437).

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### Description

Enabling the `use_insecure_ssl_client_just_for_testing_do_not_use` config file option doesn't stop SimpleHttpClient (used in the OIDC handler in my case) from failing to connect to host. It also fails with a 504 which is its way of saying "invalid certificate", which is a bug on its own, but possibly not related.

### Steps to reproduce

- set up an OIDC provider that uses a self-signed SSL certificate
- try to start up synapse
- observe how it fails to start due to `RequestTimedOutError`

With `use_insecure_ssl_client_just_for_testing_do_not_use` enabled, I'd expect to have a normal interaction even with an untrusted server.

### Version information

- **Homeserver**: locally hosted instance
- **Version**: `develop` as of the time of filing this issue, `git describe --tags` being `v1.39.0rc2-736-g1d8b80b33`
- **Install method**: N/A
- **Platform**: Pop_OS 21.04, no VMs or containers