chore: drop PostgreSQL 9.6 support (EOL since Nov 2021)

Artyom Kartasovrequested to merge
683-postgres-9-6-eol into master

Remove 9.6 CI job, version-specific config directory, test workarounds, and pg_xlog/pg_xlogdump fallback code. Minimum supported and tested version is now PostgreSQL 10.

Closes #683 (closed)

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