Adjust code for PostgreSQL 12 EOL

[shruthikc-gowda]

The below check in pg_squeeze.c needs adjustment since we no longer support Postgres 12.

#if PG_VERSION_NUM < 120000
#error "PostgreSQL version 12 or higher is required"
#endif

[ahouska]

Thanks @shruthikc-gowda for pointing that out. PG 13 will be EOL in November, so I'll fix this issue when removing support for PG 13.