update Dockerfile for pg_auto_failover/docs/tutorial

[RMTT]

Current Dockerfile use postgresql 14 and citus 11.1.2 which will cause following errors when build:

• /usr/src/citus/src/include/pg_version_compat.h:59:1: error: redefinition of 'RelationGetSmgr'
• /usr/include/postgresql/internal/libpq-int.h:86:10: fatal error: common/pg_prng.h: No such file or directory

The first error has been fixed since citus 11.1.6 and the pg_prng.h exists since postgresql 15.

Update postgresql version to 16 and citus version 12.1.0 can fixed above two errors:

tutorial=# select version();   
                                                           version                                                           
-----------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 16.1 (Debian 16.1-1.pgdg110+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 10.2.1-6) 10.2.1 20210110, 64-bit
(1 row)