Update pg_dbms_stats to support PostgreSQL 15

[Lerm]

This PR aims to adjust pg_dbms_stats to be compatible with PostgreSQL 15.

It includes following changes:

• Obvious changes related to version change (excluding copyrights) - changing version of extension and referenced PG version to 15, renaming installation script.
• Adjust regression tests to removal of dynamic translation in PGSX (see commit in PG15: postgres/postgres@d1029bb). Basically it moves tests from input/output templates to ready-to-use sql/out files which uses environment variables to adjust to build location.
• Fix problem with query in 'import.c' file (function 'dbms_stats_import') which had no space between numeric literal and 'ORDER' keyword (which is now prohibited in PG15 after following change: postgres/postgres@2549f06).
• Increase sleep interval from 0.7s to 1.2s for pg_stat-related tests in 'ut-common.sql' before looking into the view 'lockd_io' to adjust for change in pg_stat from PGSTAT_STAT_INTERVAL in PG14 (500ms) to PGSTAT_MIN_INTERVAL in PG15 (1000ms).
• Synchronize content of function 'dbms_stats_estimate_rel_size' in 'pg_dbms_stats.c' with current content of 'estimate_rel_size' in 'plancat.c' file in PG15.

This is related to the question about release for PG15 version - #9