notifications: Deprecate redundant encryptions settings

'tls' and 'ssl' are duplicates of 'starttls' and 'ssltls' and have been deprecated
in the upstream modules we use for sending mail notifications. Let's deprecate them
as well and issue a warning when they are still used.

Fixes: #7345
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f