Update _migrate_self_postgres_timescaledb_compatibility.md

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Description

The current compatibility matrix specify an invalid compatibility between postgresql 13 and timescaledb 2.16

cf https://github.com/timescale/timescaledb/blob/eea28951e9eaec71be8d203afd25be7cdcb59a56/CHANGELOG.md#2160-2024-07-31

PostgreSQL 13 support removal announcement

Following the deprecation announcement for PostgreSQL 13 in TimescaleDB v2.13, PostgreSQL 13 is no longer supported in TimescaleDB v2.16.

The Currently supported PostgreSQL major versions are 14, 15 and 16.

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