This lets us:
- avoid issues with broken hash indices for PostgreSQL <10
- drop runtime checks and legacy codepaths for <11 in db search
- always enable custom query plans for performance optimisation
PostgreSQL 11 is already EOL since 2023-11-09, so
in theory everyone should already have moved on to 12 anyway.
Prior to this change, anyone, authenticated or not, could submit a search
query for an activity by URL, and cause the fetcher to go fetch it. That
shouldn't happen if `limit_to_local_content` is set to `:all` or if it's
set to `:unauthenticated` and the query came from an unauthenticated
source.
