Document use_legacy_storage pages config option

What does this MR do?

See omnibus-gitlab#5993 (closed), this MR:

1. removes deprecation notices about domain_config_source as we already removed support for it in 14.0
2. simplifies domain_config_source description down to a minimum and mentiones that it's available only from 13.3 to 13.12
3. documents use_legacy_storage which replaced domain_config_source

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Does this MR meet the acceptance criteria?

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