feat(search): swap plainto_tsquery for websearch_to_tsquery

[viq]

This switches plainto_tsquery to instead use websearch_to_tsquery, introduced in PostgreSQL 11. With unquoted text it behaves the same, but allows to use quoted text and OR and negation in the search terms.

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• I have tested my changes
• There are no breaking changes
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To the above checklist: I ran the integrated test suite, including integration tests, and everything passed.
I ran it where I'm figuring out which reader I like more, which has ~300 feeds, but I don't use that much - it does work there, and now I can perform a something OR other OR evenmoreoptions search, and it returns results (they seem to be sorted by relevance more than recency?).
On nextflux connected to this miniflux instance, a simple single word search continued to work, but the complex search with OR did not.

And I will point out that I don't know enough to known the implications of this, I don't know what this can affect and where, and used a simple search-and-replace on the instances of the function. But, this seems to drastically increase functionality of available search.

[jvoisin]

The documentation says that PostgreSQL ≥ 9.5 is required. Maybe it's time to bump this requirement?

Debian bullseye (oldoldstable) ships PostgreSQL 13.