Align nightly cassert PG minors to main (17.11/18.6)

[ibrahim halatci (ihalatci)]

What

Aligns the nightly cassert workflow's PostgreSQL minor versions with main.

citus#8804 bumped the canonical CI coverage in build_and_test.yml from 17.10 / 18.4 to 17.11 / 18.6. This PR carries the same minors into .github/workflows/nightly_cassert.yml so the nightly builds the same PG minors the PR pipeline targets.

Changes

Three params lines in the params job (single source of truth), bumping 17.9 → 17.11 and 18.3 → 18.6:

• pg_versions
• pg_upgrade_pairs
• citus_upgrade_pg

PG16 (16.13) is intentionally left unchanged — main sets no PG16 minor in build_and_test.yml (PG16 support is being wound down), so the nightly keeps its existing 16.13 pin. citus_old_version and everything else are untouched.

Notes

Companion to the arm64 nightly minor alignment in citus#8816 (that PR bumps nightly_arm64.yml to the same 17.11 / 18.6). Kept as a separate PR since it touches a different, already-shipped workflow.

Opened as a DRAFT.

Refs citus#8804.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (92d0b92) to head (0392204).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8822      +/-   ##
==========================================
- Coverage   88.73%   88.73%   -0.01%     
==========================================
  Files         289      289              
  Lines       65013    65013              
  Branches     8203     8204       +1     
==========================================
- Hits        57691    57688       -3     
  Misses       4954     4954              
- Partials     2368     2371       +3     

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