feat(settings): implement support for 'silent_mode_enabled'

[AdrianDC]

Changes

Implement support for https://docs.gitlab.com/administration/silent_mode/

Example:

Documentation and testing

Please consider whether this PR needs documentation and tests. This is not required, but highly appreciated:

• Documentation in the matching docs section
• Unit tests and/or functional tests

gitlab application-settings update --silent-mode-enabled 'true'
gitlab application-settings update --silent-mode-enabled 'false'

• Documentation automatically generated and new key matches signup_enabled sources
• No test added as most other settings are not implemented in tests/functional/cli/test_cli_v4.py

[JohnVillalovos]

Thank you.