Add guide about migrating the Invidious database to a new version of PostgreSQL for users using Docker. #730

Since PostgreSQL is going EoL, we should dump the PostgreSQL version to the latest one (18) if possible, giving some simple instructions and commands on how to do it without losing data.

Hi, postgresql 14 will be EoL in about 2 weeks. Is it being considered migrating to a supported version?

Originally posted by @g1sm in #601