Drops PG15 support

[naisila]

DESCRIPTION: Drops PG15 support

Steps involved:

• Remove pg15 from configure

• Remove PG_VERSION_15 lines

• Delete ruleutils_15.c

• Remove PG_VERSION_NUM >= 16 and PG_VERSION_NUM < 16

• Remove pg_version_compat.h '>= PGVERSION_16' entries

• Clean up server_version_ge_16 from tests

• Remove pg15 CI tests

• Remove normalize rules specific to PG15 diffs

• Rename pg_get_object_address_13_14_15.c to pg_get_object_address_16_17_18.c

• Remove PG15 obsolete comments

• Remove PG15 from pseudoconstant qual error

Note to reviewer: You can review commit by commit, the commit message should be explanatory enough. You can ignore the flakyness failures.

Fixes #8351

Sister PR citusdata/the-process#178

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (0e110ee) to head (4a5be6f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8372      +/-   ##
==========================================
- Coverage   88.94%   88.73%   -0.21%     
==========================================
  Files         287      287              
  Lines       63153    63019     -134     
  Branches     7939     7891      -48     
==========================================
- Hits        56169    55922     -247     
- Misses       4671     4781     +110     
- Partials     2313     2316       +3     

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[eaydingol]

Thanks, looks great.

In a few places, some comments seem to be obsolete after pg15 support is dropped, like deparse_table_statements 181-183, adaptive executor look for PG15+, recursive planning 531-533 (no need for pg15 there), multi_logical_replication function comment.
Please review these and update them as you see fit.

[eaydingol]

Is there a historical reason to name the file _13_14_15? , If this is related to the supported pg versions, can we update the file name and maybe the comment in it?

[naisila]

Good catch, I will rename the file. The header itself says

 * We need to make sure that function works with future PG versions. Update
 * the function name according to supported PG versions as well.
 *