Adds Citus compatibility with PG18.0

[m3hm3t]

Adds Citus compatibility with PostgreSQL 18.0 by wiring PG18 into the devcontainer, configure checks, and CI matrices.

Summary

• Build PostgreSQL 18.0 in the devcontainer image via pgenv and stage its artifacts alongside existing PG15–PG17 installs.
• Switch the default devcontainer PostgreSQL version to 18.0 so local developer workflows exercise PG18 by default.
• Extend configure / configure.ac version checks to treat PostgreSQL 18 as a supported server version.
• Update GitHub Actions build_and_test.yml to:
  • Define a pg18_version entry (major: 18, full: 18.0).
  • Add PG18 to all relevant test matrices (check-split, check-multi, regress, enterprise, pytest, CDC installcheck, and query-generator).
  • Include upgrade paths to PG18 (15→18, 16→18, 17→18) in the upgrade-test matrix.
  • Run the flakiness job against the PG18 fail-tester image.
• Update packaging-test-pipelines.yml so the supported PG version list is derived more robustly from build_and_test.yml (no assumptions about version suffixes).
• Extend src/test/regress/citus_tests/common.py with the OLDEST_SUPPORTED_CITUS_VERSION mapping for PG18 to keep upgrade tests working.

Testing

• Devcontainer image builds successfully and pgenv switch 18.0 works inside the container.
• GitHub Actions:
  • build_and_test.yml matrix includes the new PG18 jobs and they complete successfully.
  • packaging-test-pipelines.yml still discovers the expected set of PostgreSQL versions.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 88.79%. Comparing base (6d15669) to head (7ce775e). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8065      +/-   ##
==========================================
+ Coverage   88.74%   88.79%   +0.05%     
==========================================
  Files         287      287              
  Lines       63019    63112      +93     
  Branches     7891     7908      +17     
==========================================
+ Hits        55926    56042     +116     
+ Misses       4777     4745      -32     
- Partials     2316     2325       +9     

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