Config: Rename "imprint" option to "legal-info"

[lastzero]

Make sure to use PHOTOPRISM_LEGAL_INFO and PHOTOPRISM_LEGAL_URL once this has been released.

This is a follow-up issue to improve the naming of the config option that was introduced to satisfy this feature request:

• Config: Add option to display legal info in the footer, e.g. the ICP number #1990

Acceptance Criteria:

• Since "imprint " is mainly used in Germany, the config option should be renamed to "legal-info" to be more international
• Likewise, "imprint-url" should be renamed to "legal-url"
• If the new values are not set, the old naming will be used for a limited time and a warning will be issued about the deprecation