Remove matrix-bridge-mautrix-hangouts role

[luixxiul]

Is your feature request related to a problem? Please describe.
As Google Hangouts was discontinued on November 1, 2022, matrix-bridge-mautrix-hangouts role should be removed from the playbook to reduce the tech debt.

Describe the solution you'd like
Remove matrix-bridge-mautrix-hangouts role.

Describe alternatives you've considered

Additional context

[spantaleev]

I could remove the role right now and add detection for remaining matrix_mautrix_hangouts_* variables in people's vars.yml, telling them the bridge is removed.

However, that doesn't easily let them uninstall it. To let uninstallation be handled by the playbook, we need to keep the existing Ansible role around.

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For this reason, I've made the playbook show an error message to anyone who has this bridge enabled, telling them about the role-removal plan.

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I have a reminder for February 2025 to actually remove this role. At that point, I plan to:

• remove the mautrix-hangouts Ansible role and all references in the rest of the code (group_vars/matrix_servers, setup.yml, etc.)

• tweak our mautrix-hangouts docs page further (likely removing everything below the initial introduction and only keeping some "This component existed, but doesn't anymore.." tombstone)

• add validation tasks in roles/custom/matrix_playbook_migration/tasks/validate_config.yml which detect any remaining matrix_mautrix_hangouts_* references in people's vars.yml configuration file and tells them to remove them, as well as possibly telling them how to uninstall the service manually

[luixxiul]

"This component existed, but doesn't anymore.." tombstone

docs/configuring-playbook-bridge-mx-puppet-skype.md could be used for it.