fix: replace timezone function call with view

[ansg191]

The pg_timezone_names view was added in 8.2. It should be equivalent to the function query. (Identical on postgres 9.5 and 17) See: https://pgpedia.info/p/pg_timezone_names.html

This small change allows miniflux to run on postgres-compatible databases like CockroachDB, which don't have this function.

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